And it shall come to pass that everyone who calls upon the name of the Lord shall be saved. (Act 2:21 ESV)It is false to think that calling out to God is all one needs to do to be saved, a necessary requirement does not imply it is the only requirement. However I am not certain this verse is even showing a condition of salvation, rather it is showing God's faithfulness. If we call out to God he hears us and is faithful to answer our call (according to his promise). Peter in his sermon also said:
"Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. (Act 2:38 ESV)This verse shows the importance of repentance. One must turn from his own way and choose God's way. It is not possible to be part of the kingdom and walk one's own path. Yes we fail at times, and we ask for forgiveness, but our desire is to do Christ's will and not our own.
So what of John 3 which clearly talks about salvation:
"For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life. For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God. (Joh 3:16-18 ESV)Likewise Paul and Silas said to the Philippian jailer:
"Believe in the Lord Jesus, and you will be saved, you and your household." (Act 16:31 ESV)How we understand these verses in based on what does it mean to believe. The book of Acts is giving a summary verse of Paul's response to the jailer's question, "What must I do to be saved?" We need to study scripture thoroughly to understand the full implications of belief.
Jesus said to Paul that he was sending him to the Gentiles...
...to open their eyes, so that they may turn from darkness to light and from the power of Satan to God, that they may receive forgiveness of sins and a place among those who are sanctified by faith in me. (Act 26:18 ESV)Here we see that Jesus talks about being made holy by faith in him. Although it is Jesus who makes us holy, he sees this completely congruent with us turning from darkness to light.
Prior to his ascension Jesus commands the disciples:
Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age." (Mat 28:19-20 ESV)Jesus tells them to make disciples and what that entails; baptising them (for the remission of sins) and teaching them to observe his commandments.
This is consistent with the meaning of believe which encompasses the concept of trust, faith and faithfulness. Belief in Christ is more than mental acknowledgment of his existence, even the demons do that, it is choosing a lifestyle of obedience, it is becoming a follower of Christ.