Have mercy on me, O God, according to your steadfast love; according to your abundant mercy blot out my transgressions. Wash me thoroughly from my iniquity and cleanse me from my sin.
Christianity tells people to repent and promises them forgiveness. It therefore has nothing (as far as I know) to say to people who do not know they have done anything to repent of and who do not feel that they need any forgiveness. It is after you have realized that there is a real Moral Law, and power behind the Law, and that you have broken that law and put yourself wrong with that Power—it is after all this, and not a moment sooner, that Christianity begins to talk.
Prayer: Lord Jesus, help me to recognize my sins and to ask forgiveness.