It’s based on a few different ideas depending on which particular Protestant group you’re dealing with.
One of them is a form of strict penal substitution. This is the idea that Christ
literally suffered Hell for us. In other words, Jesus wsa seperated from the Father and literally went to Hell, the place of the damned. Now because He is God and thus infinite, spending even a moment in hell is equivalent to all the humans that ever sinned spending an eternity there. When a person accepts Christ, then that person gets a sort of “check mark” in “God’s payment ledger,” saying Christ paid for all his sins. It doesn’t matter what that person will do in the future, because Christ already suffered all that penalty for him.
Another is the more general concept of imputed righteousness. In almost all Protestant systems, a person who comes to Christ doesn’t really become just. God simply declares that person just because when they accept Christ, they get the right for Christ to represent them at the judgment. When they die, God doesn’t judge them exactly, but rather, He looks at them and sees Christ, so anyone who has accepted Christ is judged not by his own life, but by Christ’s life.
The problem with the first idea is that if the Son could ever be seperated from the Father, you would have two gods, not one. God would not be a three in one Trinity, but would be at least two gods. The whole idea of the Trinity is that God is only one. One being cannot be seperated from itself. Folks that believe this idea either don’t recognize the huge problem in their belief, or just claim it’s a mystery human beings can’t understand. The problem here is by and large a very weak understanding of the Trinity. Without getting into it all, the belief in the Trinity is not one that is just blankly against reason and we can’t understand at all. Rather, it’s something we can never understand
completely, but we certainly can understand the logic of it to some degree, and both modern Christianity and the early Church fathers are full of an understanding of the Trinity that begins to pull away the shades of mystery. It’s not even remotely the same sort of thing as saying the Father and the Son seperated from one another. That is to say that there is no way to understand the mystery at all, whereas the real mystery of the Trinity can be understood to a point. We can at least start to see how it makes sense. The concept these Protestants have, by the way, completely contradicts all the understanding of the Trinity that we
do have, and that which the Fathers of the Church taught us.
The problem with the second theory is all sorts of teachings from Scripture which are too numerous to get into here, but which you can see examples of here:
scripturecatholic.com/justification.html, particularly #3.
God bless.