The first Gospel sermon contains important information, but what remains unspoken is just as important.
First, in Acts 2 Peter doesn’t tell the sinners in Jerusalem to “just believe” in order to be saved.
After explaining to the Jewish audience that they had killed their Savior, but that God had raised Him from the dead and set Jesus at His own right hand in heaven, some in the crowd ask what they need to do (Acts 2:37). In verse 38, Peter tells them to repent and be baptized so their sins can be forgiven.
He doesn’t tell them to believe because they already believe. Peter has already preached Christ to them, and they respond by asking what they need to do. They’ve accepted Peter’s information as true.
According to the popular Protestant doctrine of “faith only,” handed down by 16th century reformer Martin Luther, these people in Acts 2 should have already been saved. They believed in Jesus. By the time Peter opens his lips in verse 38, they have accepted the basic facts of the Gospel. But they aren’t saved yet, because Peter goes on in verse 38 to tell them what to do to be saved. A sinner is not saved at the moment he believes.
Second, Peter doesn’t tell this group of believing sinners to pray, even though a lot of people today are told to pray for Jesus to save them.
People usually cite Romans 10:13 when telling others to pray for salvation: “For whosoever shall call upon the name of the Lord shall be saved.” But they could just as easily cite Acts 2:21, since Peter says the same thing.
At this point in the first recorded Gospel sermon, Peter is explaining that the miraculous sights and sounds the Jews in Jerusalem see and hear are the fulfillment of an Old Testament prophecy. Part of the prophecy is that “whosoever shall call on the name of the Lord shall be saved” (Acts 2:21). In other words, it’s time for salvation to be preached and for sinners to be saved.
Yet when believing sinners ask what to do in verse 37, Peter does not tell them to pray. In verse 38 he tells them to repent and be baptized to be forgiven. The salvation that was to come by calling on the name of the Lord in verse 21 is explained in verse 38 as repenting and being baptized in the name of Jesus Christ.
No one is saved by simply believing or by praying. To contact the blood of Christ and have your sins forgiven, you must believe, then be willing to repent and be baptized, as Jesus commands.