Not following the parents religion.
For those ladies who are religious, if your child chose not to follow the religion you are bringing them up as, would you be very upset?
I was brought up Catholic myself but have not kept with that faith. Everyone told me my grandmother would be very upset, but it turned out that she didn't care very much. She was happy that I still had faith and told me I had to figure out my own religious path. I was always super thankful that she was understanding.
That said, I can see how it is upsetting if your child doesn't follow what they were raised with.