I don't see this as a FU. I too am somebody who came from a religious background, though not particularly strict, and fell away from religion at a young age. It sounds, from the way you tell it, like your mother is worried 1. about what your father will think, and 2. about your "soul."

I don't have much to say about if your family can't accept you for who you are and what you believe. The real truth is that if they don't, you're going to have to make some serious decisions at some point. But as far as your soul is concerned, I think things can be a lot more palatable to your family if you can show some appreciation for some of the social customs that go along with your religion and some of the good memories you've had. You mention having had a good childhood and good family. That's great! You can still celebrate Ramadan and Eid and any of the other Muslim holidays I'm not familiar with with your family, the same as I can celebrate Christmas or Easter or whatever. And you can have a lot of fun and still build plenty of memories just as a matter of tradition. That doesn't mean you have to stick with some of the more overbearing traditions that some Muslim sects have or actually have any beliefs.

I expect that once your mother (and possibly father) have the opportunity to digest this change, you'll be okay. I would expect to be harassed about finding God. Trust me, I've heard that one from the older folks in my family for 20 plus years.