Because God is outside all space and time, what He did on Calvalry 2000 years ago goes back to the first man. That is, whatever the first sin was, Jesus not only atoned for it, but, bought them back to own to give to God the Father. The rest of us, who die after Christ are on credit. Jesus Christ bought us, but, we are continually paying back to God the one time payment.
Christ paid for us in advance because if it was just the one time in and out of history Jesus was sacrificed, He could have only bought back all those who had died before Him, but because He died once in history and is sacrificed throughout and outside of time, He got, is getting, and will get those who were, are, and will be.
He couldn’t have bought us because we didn’t exist on Earth before Him, and we were nothing to buy. He bought those before him because they did exist but were not owned by God. We are now bought but even though we are on credit and making payments to God, we are also on layaway because He doesn’t have us fully yet.
If somebody has wronged another, not only have they wronged this person in the image and likeness of God, but, they have wronged God. That person cannot go back in time to undo what they did, the most they can do is apologize or do something more to the thing already done, whether positive or negative. It is because we cannot go back in time to repair the sins we have caused in our life, the perpetual Sacrifice of the Mass heals what we cannot go back and change.