My MIL is ostentatiously Christian. More Christian than you could ever be. More Christian than the Pope. Certainly, more than I could be.
During my Wedding, she loudly talked, during our vows, about her 'coming to Christ'. She actually stopped my ds' Christening by holding her hand up and doing similar (she was then furious that she didn't get The Picture of her and him in his robe as he became ill and had to be taken to the hospital in a babygro which was underneath & easier)
When it came to the time for my Dds Christening, MIL declared it to be 'at the wrong time of year, why not hold it in the summer when the flowers are our for nicer pictures'. I'd had a lot of infertility & misc and it was important to me at that time to hold the Christening within 6m of Dd birth, so we did and MIL didn't attend, in very high dudgeon.
She has 5 grandchildren. 4 Christening pics on mantelpiece as I've never 'remembered' to give her one of Dd's out of pettiness (nor has H, out of laziness). I am aware this is hardly Christian behavior of me either and I can't think of another time I've deliberately done similar.
(it doesn't upset Dd as the children are never invited to visit. MIL is more about the pictures to show pals than the children themselves)