2 Corinthians 5:19
It was God [personally present] in Christ, reconciling and restoring the world to favor with Himself, not counting up and holding against [men] their trespasses [but cancelling them], and committing to us the message of reconciliation (of the restoration to favor).
The word reconciliation means to make an exchange, to restore to favor with God. Jesus became our substitutionary sacrifice by being made sin so that we could be made righteous. What Jesus is we have become, for He became what we were. Once born again we can stand in the presence of God without shame, for we have become one with Christ. God was in Christ completing this work on the earth. In the eyes of God every sinner got saved at Calvary when Christ became sin, therefore God is not keeping a record of the sinners sins.
This is what “not imputing” their sins means. What will send a sinner to hell and eternal separation from God is not receiving Christ as their Savior. It is the sinful nature within that is the root of the problem that has them separated from God. All who are born again now have a ministry of sharing this with sinners. It is not their sins that is keeping them separated from God, but the sinful spirit within. Their sins are the result of that sinful nature.
So when you witness to a sinner don’t focus on their sins, focus on the sinful nature within them for that is the root of the problem. If they will repent (turn to God) they will have that sinful nature removed and they can experience reconciliation, the great exchange, dying to the old man and receiving a new spirit within, and thus becoming born again. This is the word about reconciliation He has committed to the church to share with the whole world.