Why is believing in God and/or religion always such a debate?

So I’m interested in knowing how come believing in God or a particular religion ends up becoming debatable to some certain people not just here on fluther, but I experienced it the other day with a family member.

I state, I believe in God, I do not believe in any one particular religion and it always gets challenged. I’m not out to change anyones views so why try to change mine and why can’t people just let it be?