Well, I don't think this is hate mail. It's more "concerned-for-our-immortal-souls" mail. I appreciate this person's well wishes and am sorry that they worry about such things. It must be quite a burden.
Forgive me for what? I haven't done anything wrong. Even if somehow I'm wrong about god, it doesn't mean that person is right. And any god that would punish someone for a very reasonable opinion is not a god who deserves my worship.
And as for you my devout Christian friend, in the event that any of the other countless gods conjured up by humans is by chance the one true god (like zeus for instance), then i hope he'll forgive you for choosing to follow this "jesus" dude