I am a Christian but I have sinned . I feel guilty. What can I do?
If you know what your sin is, and you feel guilty, thank God for it. This is God's way of helping you to see the need for change in your life. The good news is that you can be forgiven by God. Jesus has already paid the price for your sins.
God has made a provision for us to receive forgiveness of our sins. 1 John 1:9 says, "If we confess our sins, He is faithful and just to forgive our sins, and cleanse us from all unrighteousness."
The first things to do is tell God out loud what it is you did that you know was wrong. Be specific. God is not shockable. He already knows. But He wants you to tell Him so that a right relationship with God can be restored.
Do not delay confessing to God when you know you have sin in your life. Do not wait until you get into church or some spiritual environment. Confess it immediately to God.
If you know when you confess that you fully intend to do the same sin again, then it is quite possible that you are not truly born again. You need to forsake yourself and truly give yourself to Christ. Check out this lesson on salvation and this lesson on repentance.
If your sin was against someone else you will need to ask God to lead you as far as confession and restitution towards that person should be done. The sooner you do all you can in your power to confess your sin against that person to them the sooner you will remove Satan's possibilities for accusing you and bringing you down.
If you sincerely desire to be in a right state with God, and have confessed your sin to God, then believe God's promise that you are forgiven! If you still feel vaguely guilty but you are not sure why, remember that God says, "There is therefore now no condemnation to those who are in Christ Jesus, who walk not after the flesh but after the spirit." (Romans 8:1). Rebuke the evil spirit of condemnation that attacks you in Jesus' name. Keep drawing near to God so that He can shine more light into your heart, so that you can make further adjustments where necessary. But go to God with an attitude that you are already accpeted and loved by Him.
God has forgiven you and forgotten your sin. But you have not, and neither have the demons who saw you. You need to be humble enough to forgive yourself also (who are you not to if God has), and remember that your sins are forgiven and washed away. Satan may bring up old charges but he has no legal right to do so if you have confessed and forsaken those sins.
"As far as the east is from the west, so far has he removed our transgressions from us." (Psalm 103:12)