2 Corinthians 5:21 is often called the great exchange, and it works like this. Imagine your life as a moral ledger in which every action, every thought, every word and every motive is recorded. It’s pretty grim… But God takes your sin and removes it from your ledger and charges it to the ledger of the Lord Jesus Christ. The Saviour bears the penalty for all your sin. You then are left with a clean, but empty ledger. So God does something else. He takes the perfect obedience, the perfect righteousness of Christ from his ledger and transfers it to your ledger. In computer terms, you might think of this as copy and paste in the sense that the same righteousness, even though it has been given to you, is still available to every other Christian, and everyone who will come to Christ in the future. Now you have a ledger that no longer catalogues your sin, but instead bears only the record of 33 years of absolutely perfect righteousness. How can God do this? How can a just God completely wipe all sin off your ledger and replace it with the perfect righteousness of Christ? Because we are in Christ. He, as our representative stands justly charged with our sin and pays its penalty through His death. And because he is our representative, God can justly credit his perfect righteousness to us. So we can say as Paul essentially says in Galatians 2:20 that when Jesus died on the cross, we died on the cross. And when he lived a perfect life, we lived a perfect life because we are in Him.