Satan saving Constantine?
God saved him. Constantine clearly looked up and said, "I really need Your help now."(or something like that) But, my point is, he asked God. Then he comitted suicide(that being the catholic "mortal" sin) and satan came up to come get him(as you said). But having bitterness. No, no my friend. That was the salvation point. He(Constantine) knew what he was doing. By commited suicide(catholic "mortal" sin), he knew satan would come up to get him, then he(Constantine) could explain the situation and he(Constantine) would trade places with the sister. If anything, have a problem with him getting into heaven by works and not faith. Sacrifice is a work, not faith.
Over all, I enjoyed the movie. I was not to big on Gaberials choices(being as he/she/it would never do that) but, over all, loved it. Reasons: Cool visuals, great story, and they used words and phrases that I have never heard on the silver sceen. IE, salvation through Jesus, sanctification, He is the way, the truth, and the life, etc.
JS