In the religious tradition I grew up in, baptism was something you did when you were old enough to make a voluntary confession of faith on your own. More to the point, baptism was just that -- it was a sort of public confession that "I have decided to follow Jesus, no turning back." There was nothing particularly mystical about baptism; it was taking a bath to let people know you love Jesus.
My present conviction, however, is that baptism actually involves much more than that. I think a sort of super minimalist conception of baptism such as the one I grew up hearing is certainly true, as far as it goes, but it doesn't say enough. Baptism is a greater and more profound mystery than that!
Consider what Paul says in Romans 6, the most sophisticated and detailed discussion of baptism in his letters. It opens with the question: What then are we to say? Should we continue in sin in order that grace may abound? (v. 1). Why should he begin with this question? What might motivate this line of reasoning? The answer is that in Paul's theology, God is very unfair; when people sin, rather than responding in terms of equal and proportionate punishment for the sin, he provides grace and salvation! This God shows his love for us by dying for us while we are still sinners (5.8); God reconciles us to himself while we were enemies (5.10). When despicable sinners turn their backs to God and go off on their own way, his response, rather than destroying them or leaving them to be fall apart as a natural consequence of sin, is to restore them and reconcile them. In a word, where sin increased, grace abounded all the more (5.20). But if sin does not get its comeuppance with God, why then should we be holy? How do we motivate holiness with a gracious God?
The answer Paul gives has to do with baptism. His answer is that we do not go on living in sin because we have died to it, and more specifically that this death to sin has taken place during our baptism (6.2-3). Now this is very profound! What he is saying is that through baptism, our fundamental mode of being is entirely changed: whereas previously Paul says that all, both Jews and Greeks, are under the power of sin (3.9), now he says that we Christians who have been baptized are dead to sin, and that this has been accomplished through baptism. This inspires further research; how can something so simple as baptism free us from the power of sin?
When we look more closely at Paul's reasoning, we find that baptism for him has to do with union with Christ. Notice the language he uses:
. . . all of us who have been baptized into Christ Jesus were baptized into his death. (v. 3)
. . . we have been buried with him by baptism into death . . . (v. 4)
. . . we have been united with him in a death like his . . . (v. 5)
. . . we have died with Christ . . . (v. 8)
. . . consider yourselves dead to sin and alive to God in Jesus Christ. (v. 11)
Paul's reasoning appears to my mind to be this. God is gracious, but that does not provide us with motive to sin. On the contrary, we are not to sin because we have died to sin; we have undergone a fundamental change with regards to our identities, with regard to our state of being. This has been accomplished through baptism, because in baptism we were united to Christ, who died and was resurrected.
Now if we are freed from sin because of our union with Christ, this must be because Christ himself is freed from sin. Indeed, this narrative is behind Paul's reasoning. Notice what he says:
We know that our old self was crucified with him so that the body of sin might be destroyed, and we might no longer be enslaved to sin. For whoever has died is freed from sin. But if we have died with Christ, we believe that we will also live with him. We know that Christ, begin raised from the dead, will never die again; death no longer has dominion over him. The death he died, he died to sin, once for all; but the life he lives, he lives to God. So you also must consider yourselves dead to sin and alive to God in Christ Jesus (vv. 6-11).
Here the NRSV's choices of translation leave a bit to be desired. What they translate here as "our old self" (ὁ παλαιὸς ἡμῶν ἄνθρωπος, v. 6) I would sooner translate as "our old humanity." The idea is that Christ took upon himself a human nature such as our own, fallen and subject to sin and destined for death, the same as us. Torrance emphasizes this point, but so do Athanasius and Gregory Nazianzen. He takes our condition upon himself in order to redeem, sanctify, purify, and deify it! This is precisely what he does, ultimately through his death.
Here, too, the NRSV translators disappoint. The literal reading of v. 7 is: the one who died (ὁ ἀποθανών) is freed from sin. Campbell, by my lights, is right to read this as a reference to Christ, who died and therefore was freed from the power of sin in his human nature. This is exactly what Paul goes on to say later: The death he died, he died to sin, once for all (v. 10). Paul understands Christ at one point to have been under sin in some sense, the same as the rest of us; he doesn't suppose that Christ committed sin, of course, but merely that Christ's human nature had the "sin disease," so to speak -- that it was inclined in the wrong direction, same as the rest of us. Through his death, Christ was freed from this power, and in his resurrection he was given life to live for God forever.
Therefore, when we are baptized, we are united with Christ who is beyond the power of sin! Just as he has died to sin forever and lives to God, when we are baptized, we are to consider ourselves to be the same, since we are one with him. So Paul tells us: you also must consider yourselves dead to sin and alive to God in Christ Jesus (v. 11).
So baptism, for Paul, is far more than a mere public declaration that "I have decided to follow Jesus, no turning back." No, it is the way that we are liberated from the powers of sin through union with Christ!