Sometimes religion can be a sticky subject in a family. First of all, he may not be religious himself in terms of practicing religion, but that does not mean he does not believe in God. He might also hope that you will not follow in his footsteps. It could also just be that he disagrees with Atheism in general.
I don't know your father, but depending on your age and experience, he might be troubled by what brought you to this determination. The vast majority of society believes in a God, whether they are church-goers or not. What has happened to you that makes you not believe? He might see your beliefs as overly cynical and "unfounded" in someone who is still quite young.
I wish I could offer some new insight, but I agree with the previous posts. Your dad probably looks upon your decision as one of rebellion against the principles that he has taught you growing up. My dad attended church rarely as he grew older, but that didn't mean he was not a Christian or didn't believe in God. Your father might consider the rejection of his favorite football team or his favorite food as a sign of independence on your part, but the loss of your everlasting soul is a more troubling dilemma for him.
I agree with the other posters. One could look at atheism as a sign of rebellion. By rejecting the ideology of religion, you are rejecting him and what he has taught you. I would think that, more than anything, his feelings are simply hurt.
My guess is much like the second point from pohnpei above...that your dad may not be overtly religious, but that (culturally) he grew up and is comfortable in a social group that believes in God and orders life around the values and morals of the Christian belief system. You rejecting religion may feel to him that your are rejecting the culture and morals with which he raised you...therefore you are rejecting him.
Since none of us on here is, presumably, your dad, we can't know for sure, but here are some thoughts:
- Maybe he sees it as a sort of rebellion on your part. Obviously, atheism is very much a minority opinion in the US and he may feel like you are being an atheist just to be different and to show that you don't have to be like everyone else.
- Maybe he doesn't think that acting religious is important but he does think that at least believing in God is important. Lots of people only go to church once or twice a year and don't pray much, but still say they believe in God. If your dad is like that, he might think that you are endangering your soul by completely not believing in God. He might not be outwardly religious, but he might think that it's important to at least believe in the existence of God.
Those are two possibilities that I see. Good luck in working it out with him.
@response 2. What is the importance of believing in god? I'm in almost the same position. My parents are fine with my beliefs, but they say that i should atleast believe in god and just be agnostic.