How would I get my parent to understand that I don't believe in their God or anything they believe in for that matter? I've tried explaining it to them, but they still don't understand.

Unfortunately, it takes time for some parents to accept the lack of belief of their child.  It may take you correcting their assertions and beliefs over time for them to truly understand and accept that you are an atheist.

My fiance’s parents have struggled with accepting the fact that we are atheists.  Over the years, their claims that we will change our minds when we, “Grow up,” have become less frequent.  (Thankfully.  Those were incredibly hurtful and insulting, in my opinion.)  While we still run into occasions where they cling to the idea that we either aren’t atheists or aren’t going to permanently be atheists, it has become less frequent over the years.

Good luck, friend.  It’s frustrating.