Christians who have received Christ are declared righteous based on Christ’s righteousness being imputed to the believer (2 Corinthians 5:21 and Romans 3:21-26).
http://www.gotquestions.org/justification.html
For believers in Jesus, the punishment for all our sin (past, present and future) has already been dealt with through the death of Jesus. “There is now no condemnation for those who are in Christ Jesus” (Romans 8:1). Because of the sacrifice of Jesus, God sees only the righteousness of Christ when He looks at us. Our sin has been nailed to the cross with Jesus, and we will never be punished for it. However, because we are imperfect, we will continue to act in sinful ways and if we do not repent and turn from that sin, God brings His divine discipline to bear upon us to turn us from rebellion to obedience. Check out this link:
http://www.gotquestions.org/God-punish-sin.html
The Bible tells us that “If we confess our sins, He is faithful and just and will forgive us our sins and purify us from all unrighteousness” (1 John 1:9). To confess our sins to God is to admit before God that we were wrong, that we have sinned. God forgives us through confession, on an ongoing basis, because of the fact that He is “faithful and just.” God is faithful by forgiving sins which He has promised to do for all those who receive Christ as Saviour. God is just by applying Christ’s payment for our sins, recognizing that the sins have been atoned for. This is a great encouragement to every Christian.
Go to John's gospel, and read the first chapter. Dwell on verses 12 & 13 about the difference between "accepting Jesus" (the modern phrase) and "receiving Jesus" (the biblical phrase) by faith. Consider also how salvation by Jesus has a three-fold aspect. This was expressed by John Stott like this:
"I have been saved - in the past - from the penalty of sin - by a crucified Saviour.
I am being saved - in the present - from the power of sin - by a living Saviour.
I shall be saved - in the future - from the presence of sin - by a coming Saviour"