Parents want what is best for their kids. Perhaps, your dad feels like religious morals and ideals would be better for you than atheism. While he might not be outwardly religious himself, he might wish that you would follow the path he feels is best for you. I'm not saying that either religion or atheism is the better path, but that might be what your dad is thinking. Religion
can prove a lot besides just a belief in a higher power. Maybe he doesn't want you to miss out on all that religion can give a person. Remember when parents are upset about this type of thing, it usually just means they love you. Try talking to him about it calmly.