This is the fourth post on the Church in my ongoing series, theology in bullet points. The first unit in this series had to do with God and Creation (book here), and the second unit was on Christology and Atonement.
We are now in the third and final unit: The Holy Spirit and the Church. The first set of posts in this final unit on the Spirit and the Church was on the Holy Spirit.
The church is one body even though it has many members.
1. When the church at Corinth was having problems with disunity, Paul drew on a metaphor that was arguably known in the Greek-speaking world and perhaps particularly known in some way at Corinth.  Although the precise background is not clear, Paul's meaning seems clear enough, especially in the light of the Corinthian situation.
The Corinthian church suffered from significant disunity. Its key problem seems to be that some in the community thought themselves superior to others in the community. Indeed, some of them seemed to think themselves superior to Paul.
Some clearly thought themselves wiser than others (e.g., 1 Cor. 3:18). Because they thought they had superior knowledge, some seemed to revel in their superior freedom to do things others in the community did not do (cf. 8:2, 7). Some thought that they had superior spiritual gifts to others in the church, such as the gift of tongues (e.g., 12:4; 14:1-2).
1 Corinthians 12-14 is Paul's response to this problem in the Corinthian church. The various individuals in the Corinthian church are like the various members of a body. It would be absurd for the parts of a body to fight against each other because they are one body. The different parts of the body serve different functions, and they all contribute to the common benefit (1 Cor. 12:7).
The different parts of the body are not to look down on each other (e.g., 1 Cor. 12:16). We give extra honor to the parts of the body that are often overlooked just to even out the honor among the parts of the body (e.g., 12:23). "For in the one Spirit we were all baptized into one body—Jews or Greeks, slaves or free—and we were all made to drink of one Spirit" (12:13).
Paul reiterates this image of unity in diversity throughout first Corinthians. The one loaf in the Lord's Supper indicates that, "because there is one bread, we who are many are one body, for we all partake of the one bread" (1 Cor. 10:17).  The collective body of Christ at Corinth is "the temple of God" (3:16).
2. So in what does the unity of the Church consist and in what ways is it diverse? 1 Corinthians 13 makes it clear that a key characteristic of its unity is the love of the members of the church for each other. It is no coincidence that 1 Corinthians 13 appears here, in the middle of a discussion of spiritual gifts. If the Corinthian church was divided, the solution was for them to love each other.
Paul repeatedly expresses this aspect of Christian unity. Tell Euodia and Syntyche to be of the same mind (Phil. 4:2). Tell the Philippians as a whole to be of the same mind, in one spirit, in one accord (2:2). Have the attitude of Christ, who had the rank of a king, but had the attitude of a servant (2:5-7).
Paul was not speaking of unity in belief here, although he assumed that all Christians affirmed Jesus as Lord on the basis of his resurrection (Rom. 10:9). They served one God (Eph. 4:6), and Paul no doubt assumed a number of beliefs Christians held in common. But in his letters, the focus of unity is on Christians loving each other and considering each other to be of equal value.
In Romans 14 and 1 Corinthians 10, Paul makes it clear that this unity in spirit was more important than unity on the debated issues of his day. There was disagreement in his day on whether believers should eat meat offered to an idol. There was disagreement on whether Gentile believers should observe the Jewish Sabbath. Paul indicates that unity of spirit and building each other up was more important than getting the right answer on these issues.
3. Paul thus assumed that there would be diversity both in the local assembly and the universal Church. In 1 Corinthians, he assumes a diversity both of gifts and functions in the church. We should be careful not to make the various lists he gives into anything like absolutes. The letters of the New Testament are generally "occasional" in nature. That is to say, they were written on specific occasions to address specific issues. Paul did not write them--and God did not intend them--to be anything like absolute categories.
It would be absurd for a Christian to say, "I cannot help with that task because my spiritual gift is x." Similarly, it would be ridiculous for a believer to say, "I do not do that function because I am a prophet or a teacher." It is human nature to want to categorize and to pigeonhole, but that is not Paul's purpose. He is giving us examples of the kinds of gifts people have and the key roles of the early church. He was not giving exhaustive lists or absolute categories, nor was he thinking that the roles he mentioned would necessarily extend two thousand years.
For example, an apostle for Paul was a role unique to his day because to be one, the risen Jesus needed to have appeared to you physically and commissioned you to go as a witness to the resurrection (1 Cor. 9:1). In 1 Corinthians 15:8, Paul indicates that he was the last of this sort of apostle. So anyone who calls him or herself an apostle today is not using the word in the same way as the New Testament. 
Nevertheless, Paul's lists illustrate the kind of diversity of gifts among the body of Christ, as well as the various roles that individual believers often play. Romans 12:8 mentions people who are good at giving, people who are cheerful, people who are compassionate, people who are diligent. 1 Corinthians 12:28 mentions those who are good at helping others. Then there are the more showy gifts: healing, prophecy, tongues, leadership, teaching (Rom. 12:6-7; 1 Cor. 12:8-10, 28; Eph. 4:11).
The functions that take place in the church are the key, not these roles as clear cut, distinct offices ordained by God. Indeed, to make these roles into distinct positions runs the risk of giving an individual the temptation to boast that he or she is such and such a thing in the church. No personality profile or strengths test is meant to become a self-fulfilling prophecy but to describe your general gifts and tendencies so that you can function as well as possible in the world and be able to manage your weaknesses.
The capacity of human nature to take the good and use it for ill is both astounding and pervasive. How ironic it is, therefore, that many take so much pride in their supposed spiritual gifts or their supposed God-given role in the church. And how foolish it would be to limit what God wants to do through you because you have resolved that you are only an eye or that you are only an ear! That was not Paul's point.
The Church is made up of many members with many different gifts. These gifts match naturally with varying roles that we all may play in the body of Christ. But none of us are more significant in God's eyes. All of us are equally loved in God's eyes. And so should we be in each other's eyes.
The Church is one body, even though it has many members.
Next Sunday: E5. There is no one, correct form of church government or denomination.
 The classic study here is Ernest Best's, One Body in Christ: A Study in the Relationship of the Church to Christ in the Epistles of the Apostle Paul (London: SPCK, 1955). It was, for example, an image known in Stoic circles. See also Robert Banks, Paul's Idea of Community: The Early House Churches in Their Cultural Setting, rev. ed. (Grand Rapids: Baker Academic, 1994).
 The tendency of individualistic American Christians to use individual cups and wafers is a general demonstration of the fact that we have missed one of the major points of communion--oneness.
 There seems to be an assumption by some that, because the lists in Ephesians and 1 Corinthians mention apostles, that there must still be apostles today. But this is a pre-reflective reading of Scripture that does not yet know that these words were written to them first before they became God's word for us.