Hi Teke. Wish I could offer you some encouragement. The jailhouse games are never easy. I've been faced with them many time - the same stories, the same delusions. He's not sane honey. You know that.
May I suggest:
A- just writing return to sender on the unopened envelopes.
or
B- tell him the truth. tell him your glad he's found God in prison and you hope he continues to have a relationship with him when he gets out, but his relationship with you has been over for a long time. Wish him well and ask him not to send you any more letters. (and then write return to sender on the next one.) The truth is never easy. And the right choice isn't always the easy choice either. But it's for the best. For him and for you.
Whatever you do, we are here for you. But don't fall for his ********. Stay in the present. Today you are safe because he is in jail. Today you don't have to do anything but take care of yourself and your children. He's a grown man who's been messing with your head for a long time. He can wait.