I don't think he's lost his faith, as people sometimes say about the character; on the contrary, he still believes very strongly in a personal God, but he also believes that this God has been unfair and cruel to him, so he has declared war on him. Mozart isn't Salieri's enemy, he's merely the battleground.
Remember when Salieri is assisting Mozart with the Requiem, near the end? The've got to Confutatis malidictis, flamis acribus addictis. How would you translate that? asks Mozart. 'Consigned to flames of hell,' supplies Salieri. Do you believe it? asks Mozart? Oh yes, says Salieri, very sincerely.
This is pretty terrifying; he is deliberately choosing a literal Hell for himself.
Well, thank you, Strntz! It's nice to be appreciated.
In the play script Salieri refers to Mozart himself as the magic flute, which 'God blew till it cracked'. And he definitely feels and expresses pity for him.