This is the second post on sacraments 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 was on the Holy Spirit. The second set was on the Church. This third set is on sacraments.
Baptism is a sacrament of inclusion into the people of God.
1. Baptism has been part of Christian practice since its earliest days. The medium of this ritual is of course water. Its symbolism is, in the first place, that of cleansing.
However, the hearts of those who come to baptism as an adult are often cleansed on the basis of faith before they reach the waters of baptism. And those who are baptized as infants have not yet exercised individual faith and, indeed, they do not yet have sins in the proper sense to be cleansed.  The power of cleansing for sins, done by the Holy Spirit, thus does not coincide exactly with the act of baptism such that we might say that baptism is a sacrament of cleansing in most cases.
2. On the one hand, baptism is associated with the cleansing of our sins. In the book of Acts, the absolutely essential element of that cleansing is receiving the Holy Spirit.  It is the moment of the reception of the Holy Spirit when our sins are cleansed. It is this "baptism" in the Holy Spirit that is the moment of our justification and initial sanctification as belonging to God. This is the true moment of adoption as God's children, of regeneration, of redemption.
But baptism, on the one hand, acts out this cleansing symbolically in the most powerful of ways. Baptism enacts ritually our death to sin and our rising to new life. In the book of Acts, baptism and being filled with the Holy Spirit go hand in hand. In Acts, they are associated events.
But we are not literally saved through baptism. This wording in 1 Peter 3:21 is figurative. Baptism in water is an outward sign of the inward grace of the cleansing of sins. But because the "inward grace" in this case is typically a different event in time than the "outward sign," we cannot say that baptism is the catalyst for this more important spiritual cleansing (although it can be).
In Acts, baptism sometimes precedes being filled with the Spirit, and sometimes it takes place after receiving the Spirit. In each case, it is the reception of the Spirit that is the moment of cleansing. In Samaria, they are baptized and believe, but it is days before they receive the Holy Spirit (Acts 8:14-17). In the case of Cornelius, he and his company receive the Holy Spirit before they have been baptized (Acts 10:44-48), as is usually the case with adult baptism in the world today. Paul presumably was baptized by the Holy Spirit in that order as well (Acts 9:17-18).
3. Yet baptism is not simply a memorial of an event that has already taken place. It is more than a powerful symbol. Baptism has been considered a sacrament throughout Christian history. That is to say, there is a power of the Holy Spirit associated with the event that is much more than a memorial. What is this power? What is the inward grace that coincides with the event of baptism itself?
It certainly can be the grace of cleansing. That is to say, in those few cases where one's baptism coincides exactly to being filled with the Holy Spirit, what a powerful sacramental moment that is!
True baptism always involves the grace of re-filling with the Spirit. In the book of Acts, being filled with the Holy Spirit is not a one time event. It is something that can occur over and over again, especially as we face challenges that require a special "anointing" and empowerment from God (cf. Acts 4:31). We can surely say that: Authentic baptism coincides with a moment of power from the Holy Spirit, no matter what the age of the person being baptized.
4. So baptism is memorial of the cleansing of our sins. It is also a sacrament whereby we are filled with the Holy Spirit afresh. But being filled with the Holy Spirit in the early church had another "outward sign" more directly associated with it, namely, the laying on of hands. In Samaria, when baptism in water does not lead to a baptism in the Holy Spirit, Peter and John lay hands on the Samaritans so that they will receive the Holy Spirit (Acts 10:17).
We find other places in the New Testament where laying hands on a person is sacramental in this way. 1 Timothy 4:14 perhaps refers to Timothy's anointing for ministry (and no doubt an associated filling with the Holy Spirit), enacted by laying on hands. Herein is thus a precedent for ordination as a sacrament, with the laying on of hands as the outward sign.
Might we not find some divine grace that is almost always associated with the event of baptism itself, though? Might we not find some inward grace that usually coincides with the act of the outward sign itself?
There does indeed seem to be a more precise grace that God does in the moment of baptism, one that we should say involves a filling with the Holy Spirit. This is the divine grace of inclusion within the people of God. Baptism in water is the outward sign, whereby God brings an individual from outside the people of God to within his people.
5. Like circumcision for males in God's covenant with Israel, baptism ritually enacts the joining of God's people. It is the threshold of inclusion. Would we say that an uncircumcised male 6 days old was an Israelite? Well, yes, in a sense. But the act of circumcision was the outward sign that confirmed it.
In the same way, a person who has exercised faith in God is already justified, is already "saved." But the act of baptism is the outward sign that confirms it and enacts the crossing from outside to inside the people of God. In this way, an infant who is baptized is seen as part of the people of God until they remove themselves. His or her inclusion is something to be lost. 
The act of baptism thus enacts coming from the wilderness into the camp. It enacts the transition from "not my people" to "my people." It is a sacrament of inclusion within the people of God.
This is not merely a memorial of this transition. There is a change that takes place, and the person baptized experiences this change when the baptism is authentic. It is a change that involves the Holy Spirit. Any sin is cleansed. There is an empowerment for righteousness and service.
5. Faith is always involved in sacramental baptism, although in the case of an infant it is the faith of the community that baptizes. In adult baptism, the faith of the "baptisand," the person being baptized, is essential for the baptism to be sacramental, for there to be actual change and transformation in the act. In the case of infants, the faith of the community invokes the power of the Holy Spirit over the child to insulate it from the power of Sin as he or she reaches maturity. It is a faith that commits to protect the child from the world, to which the Holy Spirit responds with power.
Baptism is thus a sacrament of inclusion into the people of God. It remembers the faith and cleansing of the Holy Spirit that has usually already taken place (or looks forward to it). But it is a moment in which the Holy Spirit fills the person being baptized. Any sin is cleansed on the basis of faith. Power is received for righteousness and service. Most of all, in baptism God leads us across the threshold into the company of God's people.
Next Sunday: SA3: Christians have varying perspectives on baptism.
 In its proper sense, in the sense of what brings condemnation, sin involves intentional wrongdoing. An infant does not yet have a mature enough will to do wrong intentionally. While it is common, especially among older Christian traditions, to think of a baby's original sin being cleansed in baptism, it is ultimately problematic to think that any guilt can attach to us on the basis of Adam's sin or anyone else's sin. We sin like Adam. The power of Sin has control over the world as a consequence of Adam. But we did not sin in Adam, an Augustinian misinterpretation of Romans 5:12.
 Prior to Christian baptism, both the ritual washings of Judaism and the baptism of John the Baptist were sacraments of cleansing. For Jews before Christ, the ritual washings enabled a person to participate in the temple worship. These were often a matter of external purity, although no doubt God met many a Jew with his grace in those moments.
The baptism of John the Baptist was not yet Christian baptism, as the event in Acts 19:1-6 makes clear. John's baptism was a baptism for Israel to repent of its sins and be washed in preparation for the arrival of the messianic kingdom. No doubt the Holy Spirit met many of those who were baptized under John with cleansing. Indeed, the Holy Spirit anointed Jesus for ministry when he participated in John's baptism (although no cleansing was necessary; Matt. 3:13-17).
 Rather than the unpleasant limbo we often leave such children. By denying an infant baptism, we say they are outside the camp in the wilderness for years. We take the first seven days of an uncircumcised male Israelite and stretch them out to a decade or more. They are strangers in our midst in church, present but not belonging.