It is not only the height of arrogance but also the pinnacle of ignorance for anyone to insist they have the answers when it comes to faith or belief.
Anyone who says they know God exists and what He expects from us is mistaken. No one knows if God exists because there's no proof and there's not supposed to be. The whole definition of religious faith is to believe in something without requiring proof. If you "know" something exists then there is no need to have faith in it because you already know it's a fact. The whole point is to believe in something for which there is no proof. It's a little something called a "leap of faith". So if you believe there is a God then you do not require concrete proof; you have faith.
Consequently, those who say they know God does not exist are also mistaken. You don't know that either. You can either have faith in a God or not but you can't "know" it either way. Actually, and I hate to be the one to break this to you, but you don't know a lot of things. Neither do I. I just knew I had enough money in my bank account when I wrote that check -- until it bounced. I was mistaken when I thought I "knew" different. Scientists "knew" that the prehistoric coelocanth was extinct -- until some fisherman found one. You may think you know I'm wearing black pants -- until I turn the lights up higher and you see they are actually dark blue. Sometimes you'll be right but sometimes you'll be wrong. Just as you can't "know" that there is no other form of life on some planet far across the galaxy, you also can't "know" that there is not a God. I myself know enough to know that I don't know anything. And neither, my friend, do you. Saying that you "know" there is a God is just as arrogant and ignorant as saying that you "know" that there is not.
This is not to say that one should not have personal beliefs, religious or otherwise. Of course one should. One should keep them and hold them dear. We should all just remember that we are flawed human beings and these are "beliefs" not proven facts. So, since you can only "believe" these things are true and never really "know" it, I would advise everyone to believe whatever you want to believe but stop trying to ram your beliefs down everyone else's throats. You don't know it all and neither do I so get over yourself. If your neighbour wants to worship a head of lettuce named Howard; what's it to you? Or me? If, in your belief system, that means your neighbour is damned and will not go to heaven -- again, what's that to you or me. Who wants a crowded heaven? And, perhaps, when you and I both die, and we wake up to find ourselves in a great celestial salad bar of an afterlife presided over by Howard the Beatific Lettuce -- it might finally dawn on us that "Hey! Maybe I didn't know it all."