At the risk of being the wet blanket I think kids are best off when their parents don't undermine each other. Doesn't mean you should give her what she wants but I would suggest bargaining with her over how to handle your and mommy's religious disagreement. You don't try to convince him his mother is a fool and she doesn't try to convince him that you are evil and damned. Of course she isn't likely to go for it. I'm pretty sure you will win the battle for his "heart and mind" if it comes down to that.
Do you have shared custody with her? I'd be a little concerned about what she might subject him to if she decides to go to war over this.
Do you have shared custody with her? I'd be a little concerned about what she might subject him to if she decides to go to war over this.