"From that time on Jesus began to preach, 'Repent, for the kingdom of heaven is near'" (Matthew 4:17).
In the New Testament, the word repentance means to change one's mind or purpose, always for the better. Repentance is to acknowledge sin and turn from it.
Without first repenting there can be no forgiveness: "Peter replied, '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. The promise is for you and your children and for all who are far off -- for all whom the Lord our God will call'" (Acts 2:38-39).
Repentance is an ongoing requirement in the life of a believer. When sin manifests, repentance is required: "If your brother sins, rebuke him, and if he repents, forgive him. If he sins against you seven times in a day, and seven times comes back to you and says, 'I repent,' forgive him" (Luke 17:3-4).
True repentance produces fruit in the life of the believer; failure to repent causes loss: "Yet I hold this against you: You have forsaken your first love. Remember the height from which you have fallen! Repent and do the things you did at first. If you do not repent, I will come to you and remove your lampstand from its place" (Revelation 2:4-5).
Repentance requires that one acknowledges his sin. Repentance also requires one to be willing to turn from sin. It is an essential doctrine of historic Christianity.
"The Lord is not slow in keeping His promise, as some understand slowness. He is patient with you, not wanting anyone to perish, but everyone to come to repentance" (2 Peter 3:9).