If you keep studying you find it is God that does the expiating.
Then why do both the CCC and the Council of Trent say that it's the sinner who must expiate his own sins?
First let’s make something really clear. Anyone who is saved, is saved because of Christ’s redeeming work on the cross. Period. Without that act of love, it wouldn’t matter what we did; we could not be saved. That is Catholic teaching pure and simple.
So then, again, what do you do with the idea of ex ecclesia nulla salas and the idea that sinners must expiate their own sins in Purgatory?
Now what about purgatory? Anyone who is in purgatory is already saved.
Then what is the purpose of Purgatory? The Catholic Church says it's for sinners to expiate their sins. You say it's something else. What is it?
Everyone should be grateful for purgatory because while some of us may try to do God’s will perfectly, we often fail, and unless we are perfect, we cannot be there.
But the Bible says that Christ has already perfected forever those whom He has sanctified.
Why isn't Christ's perfection good enough?
True love is demonstrated by giving of ourselves. So when we do good works out of love, in a mysterious way they are joined to Christ’s work on the cross
JACKPOT! Christ's work on the cross + your good works.