Repentance was preached by Noah, Jonah, Jeremiah and many others in the Old Testament. This was also the theme of preaching by John, Jesus, Peter and Paul. It is a message that applies to all, both saint (Christian) and alien sinner (I John 1:8-10).
Repentance is a requirement for any who would consider becoming a Christian (Acts 2:38; 3:19). It must also be obeyed afterward (Acts 5:1-10; 8:14-25). Most agree that to repent is the most difficult obstacle to becoming a Christian and is also the greatest challenge in the daily walk afterward. Many thousands on the Day of Pentecost heard the gospel and were “cut to their heart” (Acts 2). They were convicted of having murdered Jesus Christ, the Son of God. About 3,000 of them repented and were immersed (baptized) for forgiveness (verses 37-42).
Later we learn some did not repent and turn away from their sins. Felix was terrified at the preaching of righteousness and self control, but he was not willing to repent and change his life (Acts 24:24-25). “Being sorry” for your sins is not repentance, but it leads one to repent. Then a change of life must follow (Mt 3:7-8). Having godly sorrow works repentance, resulting in a changed behavior. This is repentance. If there is no change in moral and/or spiritual behavior, then repentance has not occurred. Those coming to Christ must “turn from sin to the Lord” (Acts 9:35). They turn from false worship and immoral living, “proving their repentance by their deeds” (Acts 26:20).