Of God and His Creatures

But the vigour of a natural constitution is of no avail for restoring life, once it is interrupted by death. And mortal sin is death. The whole question is whether mortal sin is such a death as to be, in God's ordinary providence, beyond resurrection. This point cannot be determined a priori. Apart from His promises, God is not bound ever to forgive any mortal sin. Whether He will forgive, and on what conditions, depends on His free good pleasure, and that is ascertainable only by revelation, and its vehicle, Christian tradition.

Of God and His Creatures: 3.157