I don't like how the majority of these responses are just trying to gear you up to attack or belittle his religion. How about some respect for peoples beliefs people?
If you really feel the need to tell him, just tell him and if he tries to argue with you don't get sucked in to it. You've told him what you believe, there is no point arguing unless you are actually trying to convert him or something. Be respectful of what he believes, hopefully he will be respectful of what you believe.
Beniki pretty sums up my opinion on telling people of atheism.
beniki said:
Don't bring it up.
Seriously, no point in mentioning it if it's just going to make trouble. And if he drags you out to church on Sunday, well still go. You'll be grateful for the extra knowledge later on in life. If he wants to say grace before dinner, bow your head and just be damned grateful you got a plate of food.
I mean, I don't really like romantic comedies, but I'l still watch them with my Mum, and I never had to sit down and talk to her about it. I know you could argue that religion is a more serious matter, but if you're an atheist now I don't see how much it would affect your life.
Don't make a fuss about it, and don't show it off. Just.. get on with life.
Personally I don't see much point in telling my parents I'm an agnostic atheist. It's not so much my parents that are the problem than my grandma.
I don't believe in religion but I still attend Christmas mass with my Grandma. Why? Because it means a lot to her and it's not going to kill me anyway. In fact, I think it's a peaceful and pleasant way to spend only an hour of one day. Does it really matter that your dad should know what you believe, or is this your way of sticking it to him that you are escaping from his strict rule?
EDIT: When I tell my friends I'm an atheist the response I always get was "ok, but please don't force your beliefs on me" and it always annoyed me. However, reading this thread made me realise exactly why I get this response. Because there is atheists, like the majority of the people in this thread, who feel the need to point out all the flaws in everyone's beliefs and feel so superior about it. You people give atheism a bad name.