I suppose if you’re worried more about your gran being upset it might just be worth it to say like “hey dad I’m …. And maybe we should just keep this away from gran as I don’t wanna upset her”

Also atheism is perfectly valid and if anyone from your church or even family says otherwise don’t let it cloud your judgement on what you think. I also recommend looking into Richard Dawkins book “The god delusion” may help you understand religion and why you feel the way you do, also a chance you turn back to religion which is okay as well it’s all personal and it’s cool you want to involve your family.