In Acts 2, Peter preaches that God has made Jesus both Lord and Christ. When they ask “What do we do?” it’s because they believe what Peter said. If they didn’t believe him that Jesus is Messiah, then there’s no need to ask for instructions. Then Peter gives the instructions: “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit. For the promise is to you and to your children, and to all who are afar off, as many as the Lord our God will call.”
Does this mean everybody needs to get dunked to go to heaven? Some people have thought so. Others have tried to engineer some kind of special circumstance for this audience that would no longer apply today: they were under the unique curse of blaspheming the Holy Spirit; the baptism is required because they crucified the Messiah; they were in a transitional dispensational period; baptism was for Jews, not Gentiles, etc.
But no; no special pleading is required. But you do need a robust biblical theology of baptism. If baptism is the New Covenant analog of the Flood (as Peter will later write in 1 Peter 3:21), then baptism delivers you from the judgment that is coming upon the wicked world, and delivers you into a new one, just like the Flood did with Noah. That’s not some transitional/dispensationally unique item for this moment in Acts; that’s just what baptism does.
For these specific people in Acts 2 (who were lately shouting “Give us Barabbas!”), the judgment they have coming is about crucifying the Messiah, sure. But it’s not as if (say) the Ephesian Gentiles Paul preached to didn’t have their own judgment to deal with: they “were by nature children of wrath” until God saved them. There’s always plenty judgment to go around, and the consequences of sin are always deadly (cf. James 1).
For the Acts 2 Jerusalemites, the water baptism was the Christian community in Jerusalem receiving them into itself. If they heeded the warnings of Hebrews, baptism saved their lives, because when the Jewish revolt began, the Christian community fled the city, correctly believing Jesus’ promise that it would be destroyed. If they did not heed the warnings of Hebrews and returned to Judaism, then they were swept up in the revolt and–as promised in Hebrews–suffered a fate far worse than stoning.
For the Ephesians, and for us, baptism joins us to the Christian community. For most evangelicals, that really means nothing, because most evangelicals have no community to speak of, and therefore nothing to join. It would be a mistake to read that defect in our praxis into our theology. Our sin in this matter is entirely foreign to the New Testament. The life of the Body in the NT is a thick, substantial, literally life-saving community, and that’s the backdrop for this text.
Live like the heathens outside the community, and all the judgments that fall on the heathens outside the community will fall on you: “because of these things the wrath of God is coming upon the sons of disobedience; therefore do not be partakers with them.” Join the community and come under its discipline and rule of life, and you get to skip all that. Baptism admits you to the community (just as excommunication excludes you from it.)
Take Carlos for example: when I met him, Carlos was living on the street, addicted to anything that would numb him out. He’d been badly hurt, and he’d done a lot of damage to other people too, and he was running from all of it. I led him to Christ, and then found out he was suicidal, and I’d just helped him be sure he’d go to heaven. (That’ll do something for your prayer life!) A local fellowship he was already somewhat hanging out with baptized him, and when he really joined in Christian fellowship, God’s people supported him in kicking his addictions, finding a job, finding housing, getting a vehicle. Last time he came by, I hardly recognized him, he looked so good.
Meanwhile, Jimmy OD’ed on heroin in a Burger King bathroom, another guy froze to death, another guy was murdered for a sleeping bag or something similarly stupid…you get the idea. Christian fellowship saved Carlos’ life. Real sharing of life, not standing around after church and lying to other middle-class suburbanites about your week over mediocre coffee and cheap donuts.
I know that sounds harsh. The reality is harsh. Because we refuse to share life with one another, we deprive each other of the life-giving support the Body is supposed to provide. We can’t obey the “one anothers” if we don’t really spend time together, and obeying the “one anothers” is an essential part of the Christian life. Without it, we live subchristian lives. When our fake fellowship fails to yield benefits–as of course it will–we end up with an anemic view of the community, and therefore an equally anemic view of what baptism accomplishes by bringing someone into it.