Here is now the final installment of the second chapter of my proposed book, Generous Ecclesiology. The previous posts include:
Chapter 1: The Myth of the Ideal Church
1.1 The Good Old Days
1.2 The "Platonic" Church
1.3 The Incarnated Church
Chapter 2: Getting "In"
2.2 The Holy Spirit
Now 2.4: Church Membership:
In a very real sense, everything in this chapter thus far has been about getting into the (Christian) people of God, about becoming a Christian. The people of God in this sense cannot be identified with any visible body or group. While we can safely assume that the body of Christ will always meet visibly in groups, the true church in its most significant sense is an "invisible" church. Its members presumably span most if not all Christian groups and denominations, while not all who meet in these fellowships will truly be members of the people of God.
Certainly not all Christian groups function with this understanding, even if they might give it lip service. Especially in the past, various groups have at least acted as if only those in their group would "go to heaven." Indeed, many groups have explicitly taught this idea. Thankfully, the last fifty years have seen a dramatic increase in generosity toward Christians in other groups.
For example, the Roman Catholic Church would now recognize that many non-Catholics are Christians, even if they are "separated brothers and sisters." Prior to Vatican 2 in the early 1960s, the official view was that "there is no salvation outside the church," which at that time they understood to mean only the Roman Catholic Church. Prior to Vatican 2, the Roman Catholic Church taught that anyone who was not baptized Roman Catholic could not truly escape God's eternal condemnation. Ironically, while the Catholic Church has since changed its evaluation of Protestants, many Protestants today still have not extended the same generosity to them in return. Many Protestants embarrassingly would still ask, "Are you Catholic or are you a Christian?"
Today, many if not most Christians have come to recognize that there will be far more people in the kingdom than just those who are a part of their group. To think anything different boggles the mind. It would not only require us to have some sense of God predetermining who would be a Christian, which of course some Christian groups do believe. But it would also require us to think that God made sure that such chosen people were largely born into a particular Christian group at this particular point in history. It is no surprise that the groups that believe this sort of thing today are almost always small groups.
For example, there is a particular denomination that believes you must be baptized in the name of Jesus only for your baptism to count. Further, if you do not speak in tongues, you cannot truly have received the Holy Spirit. In effect, if you have never spoken in tongues and have not been baptized in a way that rejects belief in the Trinity, you have virtually no chance of being saved.
But now consider. This particular group did not exist before 1917. Church history has left no trace of any groups prior to the 1900s who both spoke in tongues and were modalists (non-Trinitarians). It thus boggles the mind to think that those in this group who think this way could be correct about what God requires for salvation. It would mean that no one born before around 1900 was saved--and only a relatively number since!
Nevertheless, many Christians today continue to think and act like the requirements for membership in their group coincide with what God requires for a person to be a Christian. For example, any number of Christian groups require you to be rebaptized their way if you are to be a part of their group. The Roman Catholic Church still will not allow you take communion unless you become Roman Catholic.
We can often understand the reasoning behind such exclusion, and we will argue that it is not necessarily wrong for a church to have special expectations for its particular members. But it seems even more important that we do not confuse the traditions and understandings of our particular group with what God requires of a person to be a part of the true people of God, the invisible church that spans all visible denominations. Every Christian group is likely wrong on some things and right on others, and it is impossible for us to know which is which!
So a person must have received the Holy Spirit to be a Christian. That seems to be the one absolutely essential ingredient for a person to be "in" the invisible, universal (truly catholic) church. But discerning who has the Spirit and who does not is hardly something that we as human beings are able to do with complete certainty. And a local church must surely be more than a collection of individual Christians who all have differing understandings of faith and practice. Things happen when people come together with common identities and purposes. If the church is meant to do something for God, God presumably expects us to do it together.
So unless Christianity truly boils down only to one-on-one, inward looking relationships with God, churches should group themselves around commonalities of some sort. Indeed, it seems inevitable that they will do so. For this reason, "denominations"--whatever we call them--are not only potentially good things. They are virtually necessary if the church is to do anything. Groups of Christians with common vision seem essential for the church's mission to take place.
And history seems to have smashed to bits our idealistic Protestant hopes that all Christians will end up on the same page if they only follow the Bible. It hasn't happened in 500 years of earnest trying! Indeed, Protestant groups have only multiplied without measure in the attempt. The notion that following the Bible alone will lead us to a common set of beliefs and practices seems decisively disproven by history. Denominations, groups of believers with distinct beliefs and practices, would seem to be an inevitable reality, despite their faults.
Throughout history, Christians have grouped themselves on the basis of different commonalities. It is typical for many Protestants to think of such common ground only in ideological terms, as if Christians group themselves into churches only on the basis of common beliefs. But this is only one way that Christians find common ground. For example, the Anglican Church includes a great variety of belief and practice under its umbrella.  The common ground has more to do with its worship than with specific beliefs or behaviors.
Other Christian groups find their common ground in a particular set of behaviors or practices. I was curious to hear recently of a group whose theology is very similar to my own tradition but that preferred to look outside our tradition for education. It preferred to send its children to a college whose external behaviors were more in line with theirs, even though that college's beliefs differed significantly from theirs. They apparently considered certain dress and social standards more important to their identity than sharing a common theology. 
The idea of a non-denominational church thus does not really get us away from specific Christian traditions. Such churches almost always reflect a particular tradition or mixture of traditions. They baptize a certain way. They have opinions about how the leadership of the church should go and about whether women can be pastors or not. They take positions on issues of sexual practice that go beyond anything clearly indicated in Scripture.
The closest it would seem we could come at this point in history to a non-denominational church would be something we used to call "inter-denominational." There is no pure or ideal church we can or should try to pattern ourselves after when it comes to specifics. The things that most distinguish Christian groups from each other often have to do with gaps the New Testament has left behind, things about which an incarnated church must make decisions. We have to decide how our group will baptize or if we will allow for multiple forms of baptism. We have to decide what we will do when we meet, whether someone will speak, who will lead if anyone, and so forth.
And if we are to have an impact on this world, our world, we will have to make decisions that involve spiritual judgment that goes far beyond the thoughts of the early church. Does your fellowship teach against abortion? The attempts to tie a Christian position on this issue solely to biblical passages consistently involve using the Bible's words in ways they were never meant originally. There may very well be a specific Christian position on this issue, but the Bible does not explicitly give it. Concrete churches must find their way under the guidance of the Holy Spirit, hopefully with the church at large eventually reaching some kind of collective consensus.
It was this way with the Trinity. Since the 400s, the vast majority of Christians have believed that Jesus is "of one substance with the Father, begotten, not made."  But show me the verse where the Bible clearly says such specifics about Jesus' divinity! On countless other issues the invisible church has not reached such a consensus. And on some issues--like the celibacy of ministers--the consensus has almost reversed these last 500 years.
The bottom line is that much, much more is involved in what we believe and practice than the Bible alone. God apparently means for it to work this way because this is the way it has always worked. Perhaps it has to work in this way, given the fact that the gospel has to play out in countless different times and places.
All these things seem to imply that belonging to a local or denominational church will never simply be a matter of what is required to be a Christian. The more "inter-traditional" a group is on a particular issue, the less clear its identity and purpose will be. So a group makes decisions about its understanding of Scripture. A group makes decisions about how the gospel plays out in its context. The result is a sense of membership in that group that is perfectly legitimate, even though it is not the same as what God requires for a person to be "in" the true Church.
The tension between "what is essential" and "what my group requires" is a difficult one, especially the more established a tradition is. For example, it is almost unimaginable that the Roman Catholic or Orthodox Church will make any major changes to the major expectations and processes of belonging to them. Their practices have over a thousand years of history behind them. A newly founded house church, on the other hand, can practically invent itself however those in its fellowship choose.
In theory, a local church should probably try to include as "members" all those who are members of the body of Christ. Although we cannot always tell for certain who is truly "in" and who is not, we should probably err on the side of inclusion rather than exclusion. At the same time, this ideal will regularly come into conflict with the broader church's understandings of proper Christian belief and practice, especially if that local church belongs to a network of churches in a particular Christian tradition.
What if, for example, a collection of churches traditionally questions whether tongues as they are practiced in charismatic churches are the tongues of the New Testament? Then let's say a tongues speaker begins to worship in this denomination. Let's say there is no question that the tongues speaker is a true believer. Then he or she is a member of the universal church and should be considered truly "in" the local congregation. But at the same time, the church leadership wishes to guard the understanding of their broader church tradition as concerns tongues. And perhaps they have reason to think the practice of tongues in their worship would create great division in the congregation.
Of course to a charismatic, this example seems to involve ignorance of what the Bible teaches on tongues. But every group has its blind spot. Paul lays down a rule in more than one place that the unity and edification of the church trumps my freedom to act in accordance with what I think I know (1 Cor. 8-10; Rom. 14). Church splits are an all too common feature of Protestant history, as disagreements over the meaning of Scripture have played themselves out time and time again.
What if a church tradition in a particular context does not believe a Christian should drink alcohol in that context? Then what if a believer joins them who is clearly a Christian but only drinks moderately? The person is "in" the body of Christ and should be considered "in" the local congregation. But at the same time, the church leadership wishes to guard the understanding of their broader church tradition in relation to drinking and discourage its members from following this person's practice.
Again, to many Christians the whole question seems silly. Didn't Jesus turn the water into real wine, not unfermented grape juice? Although they are almost certainly wrong, many groups don't think so. And even beyond this biblical question, there is the question of living out the gospel in a new context that is much different from the biblical ones. Playing out the gospel in a new context not only means that at times we will be able to do things various groups in the Bible were not able to do. But it may also at times mean we should not do things various groups in the Bible did. 
No doubt we could find some issue that creates these sorts of conflicts for just about any Christian tradition. What should a Baptist church do with a person worshipping in their fellowship who was baptized as an infant but does not want to be rebaptized? What should a Pentecostal church do with a person worshipping with them who does not wish to seek the gift of tongues? These groups have their particular beliefs and practices, yet they need to consider their fellow worshipper a full member of the body of Christ.
In the end, it seems appropriate for individual churches and groups of churches to bind themselves to their particular understandings of Scripture and particular understandings of how it plays out in a particular context. They should maintain a generous attitude toward those of other fellowships and continually seek correction and guidance from the Holy Spirit. But it is appropriate for Christians to come together with common goals and purposes. It is appropriate for denominations to set up structures that will perpetuate and sustain their understandings.
In wrestling with these issues, one denomination even set up two senses of membership.  Those who fellowship with a local congregation are considered full members of that congregation, "community members," as long as they hold to the most basic Christian understandings of that group. But then the denomination expects a greater commitment to the specific beliefs and practices of its tradition if one is to be involved in church leadership or with the denomination on a level beyond the local church. They call it "covenant" membership.
Such a structure is awkward, beyond any doubt. Should we not accept anyone God has accepted? Certainly we should. But there are so many matters of Christian identity and purpose beyond being "in" that a church can hardly be or do anything for Christ unless it has a far more delineated identity than mere Christianity. On the one hand, there is no clear ideal or New Testament church to get back to on so many defining issues. On the other hand, an "anything goes" approach gives no clear direction or mission for the church.
This current situation has resulted from the growing awareness of ourselves at the end of the twentieth century. Christians have became more and more aware of the traditional forces on their understandings and practices. As the world flattened, it became more and more difficult for us to suggest with any credibility that our little group had everything figured out. Ours was the group that really understood the Bible.
For some, such an awareness has lead to outright pluralism, where all claims to truth are equally valid--or perhaps more accurately, equally invalid. But there is another way. We can affirm our understandings and practices with humility and love. We can believe our understandings of the church are the best as far as we know while acknowledging and loving those who feel equally convinced of other understandings. God will sort us all out in the end.
We leave this question of church membership with some basic principles that probably give us a sense of the church of the future. First, it will be humble and generous towards other churches. It will realize that it is not the Church but only a church. It is almost certainly both right and wrong on many things.
Second, it will make distinctions between what it views as clear essentials and what it sees as less clear aspects of its tradition. It will try to recognize the historical forces that have led it to what it believes and practices at that point in time. It will stick closely to things it believes are really important and sit more loosely to areas where it stands in most tension with the rest of Christendom.
Thirdly, it will network generously with other Christians and other Christian churches. It will fully acknowledge that it can learn from them and indeed can come to see itself more clearly through their eyes. It will join with other churches in common areas of purpose and vision.
Finally, it will love equally those who worship in its fellowship who agree with its tradition and those who do not. It will consider all equal members of the body of Christ, even those who might disagree with key elements of its traditional identity. At the same time it is legitimate for it to set up mechanisms by which its particular understandings and practices are preserved.
 There are evangelical Anglicans, Anglo-Catholics, charismatic Anglicans, sea of faith Anglicans who do not believe God is a being, etc.
 Then of course there are inclusive churches whose defining characteristic is their inclusiveness. No matter who you are and what you believe, you are in, as long as you do not think that anyone is out. Any belief and practice is fine except for a belief that does not allow for all different beliefs and practices. Although this approach to Christianity will no doubt be increasingly popular in the days to come, it does come at a price.
For one, it is a departure from two thousand years of Christian faith, in which certain claims about God and Jesus have stood at the center of all reality. To say that it doesn't matter whether those claims are literally true or not is a fundamental denial of them. If there is a God with certain expectations, then it does matter whether you believe them or not. If it doesn't matter what you believe, then there is not a God with such expectations. You cannot have it both ways.
 A line from the Nicene Creed of AD381.
 An easy example is polygamy, which certainly is not prohibited in the Old Testament. In the New Testament, monogamy is assumed in several places, but no prohibition is ever made of polygamy in relation to most Christians. Overseers and deacons in the church are to be "the husband of one wife" in 1 Timothy 3, which probably means one wife in their entire lifetime.
 My own parent denomination, The Wesleyan Church.