But, how is one saved from his sins so that the Lord will then add Him to the body of Christ? We have seen two answers to that question in Acts 2. First, we are told in 2:21 that "whoever calls on the name of the LORD shall be saved." Second, we are told in 2:38 that one must "repent and let every one of you be baptized in the name of Jesus Christ for the remission of sins." So, which answer is correct? In truth, they both are! One must both call on the name of the Lord and be a baptized, penitent believer if he is to be saved. Although these two answers may seem quite distinct at first glance, I believe the New Testament shows them to be connected and inseparable. Allow me to explain further.
We learn from Jesus in Matthew 7:21 that one will not be saved strictly on the basis of calling out to the Lord in prayer (contrary to a popular denominational doctrine) - "Not everyone who says to Me, 'Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of My Father in heaven." One who is unwilling to obey the Father (i.e., do His will) cannot be saved, no matter how passionately he may call out to the Lord.
We must conclude, therefore, that Peter had something more in mind than just a verbal appeal to God when he affirmed that "whoever calls on the name of the LORD shall be saved" (Acts 2:21). Peter understood that to genuinely call on the name of the Lord was to obey the Lord (which would include faith, repentance, and baptism)! If Peter had understood differently, certainly he would have responded differently to the inquiry posed to him in 2:37. He did not tell the Jews in 2:38 to verbally call out to God for salvation (as many suggest today). No, the apostle Peter explained that to truly call on the name of the Lord was to believe, repent, and be baptized for the remission of sins! His teachings here (coupled with the rest of the New Testament) cannot be rightly understood in any other way (cf. Acts 22:16).
One final observation is in order. I believe there is a difference between "calling on the Lord" and "calling on the name of the Lord." The addition of the words "the name of" draws attention to the character and work of the person whose name follows. Thus, when a man calls on the name of the Lord, he is making an appeal to the character and work of Jesus, both in time and eternity, as a basis of his plea for salvation. Those who repent and are baptized into Christ are not, by means of their actions, calling upon the water to save them. No, they are pleading with the Lord, by means of their actions, to graciously save them. Those whose faith leads them to repent and be baptized manifest their trust in the Lord, and it is by their actions that they call upon His name, desiring that He will save them by His grace through their faithful obedience (cf. Eph. 2:8,9).