Atheism is a cocky move. It's mistaken… but it's incredibly gutsy.
First, how can someone say they believe there's no God? How do they make that claim, not having searched every corner of the universe at every point in time? How did they test this hypothesis exactly? At least an agnostic is honest enough to say he just doesn't know. But an atheist has such a big chip on his shoulder that he claims for himself god-like knowledge about the existence of God. In a way, he's saying that he is like God to know the truth about all reality everywhere and proclaim with authority that there is no God.
Like I said, cocky.
An atheist actually spends more time thinking about God than most believers. The difference is bitterness. The atheist believes in God so much that he blames almost everything on him. The atheist's response is to give God the silent treatment. Have you ever shunned someone? It takes a lot of concentration to not accidentally acknowledge them or speak to them. The atheist, consumed with bitterness over the pain they've experienced (probably from random events and the very human mess that people make), can't wish cancer, child abuse, failure, and broken hearts out of existence, but hey, God is already invisible. And why didn't he step in and give me a fairy-tale happy ending?! So the atheist's plan is to punch back at God by shutting him out.
Shun God. Stew in bitterness against him. Mock him. Ridicule him. Direct your angst and rage toward him.
Go ahead and try. I only wish Christians had this much passion about God.
The only good thing about atheists is, if they confront the real sources of their bitterness, they make powerful and devout believers later on. Some of the best Christians were trying hard to be atheists when they were 20.