What did the Bishop mean when he said that Jean Valjean no longer belonged to evil and that he bought his soul from him?

In context, by allowing him to take the silver, the Bishop is pointing Valjean is the direction of redemption. He isn't merely allowing Valjean to take the silver, but rather his gift restores Valjean's sense of worth and provides him with a new beginning.

“Do not forget, ever, that you have promised me to use silver to become an honest man….Jean Valjean, my brother, you no longer belong to evil, but to good. It is your soul I am buying for you. I withdraw it from dark thoughts and from the spirit of perdition, and I give it to God!”