My little children, I am writing these things to you so that you may not sin. But if anyone does sin, we have an advocate with the Father, Jesus Christ the righteous. He is the propitiation for our sins, and not for ours only but also for the sins of the whole world. (1 John 2:1-2)
Jesus is our defense lawyer. When we sin, Jesus speaks to the Father on our behalf. He defends us, saying that we should not be punished for our sins because he has already atoned for us. He says something like this: "Father, I know that ____________ has sinned and that change needs to happen in his life. But it would be unjust for you to condemn him. You would be exacting judgment twice for the same sin - from him and from me." 1 John 1:9 says that when we confess our sins, God is faithful and just to forgive us. He us just because Jesus has already made atonement for that sin.
One aspect of our justification that many Christians fail to see is that we are not just forgiven because Christ has paid for our sins. God also treats us as if we had perfectly obeyed the law, because Christ has obeyed it perfectly for us. He is our righteousness. This is truly mind-boggling! (William Lane and Paul Tripp, How People Change, p.162-163)