I'm seeing a fair deal of religion bashing and the whole, "God is a lie," thing going on here, so I will simply say this and leave it at that.
To say that you are certain that God does not exist is to imply that you hold some sort of answer that everyone else seems to be missing; to say that you know where the universe itself came from and to imply that our science is 100% accurate. They will say there is no evidence to prove God exists because they are looking for a utopian world where all of their problems are cured by a devine being. What would be the point of living? Many athiests I know only dumped religion when they felt religion failed them. There was no, "Hey, I think I will be rational today," moment about it. They screwed up, blamed it on God, and dumped everything they once believed and started anew.
Having said that, the zealots out there give religion a bad name. To assume that a single book holds all the answers and can be interpreted literally is foolish. Every single religious text out there has been translated from language to language thousands of times, and it is inevitable that many meanings have been lost through the ages. Religions are for those who want to find a deeper meaning for their lives. While it is true that mankind has exploited all religions throughout the ages for personal gain, that is simply not what religion is for.
It is healthy and natural to question ones' religion. I have questioned mine. It's all part of understanding and learning.
I suppose I posted this to make this point as well. I hear so many athiests crying, "Respect my right to disbelieve! You offend me!" when they, in turn, cannot respect another's right to believe because they have to tell the world, "God does not exist!"
ThrobbingEgo seems to have the rational thing down pat - Not stomping beliefs, but rather able to carry on an intelligent conversation and brings up some good points.