This has the advantage that it is simple. It has the disadvantage that it doesn't explain where everything comes from.

In any case, how can you possibly be sure? It takes a great deal of faith to believe that there is no God. Some people spend years searching for God and are still not sure, yet other people seem to make very little effort to find Him and still find it easy to conclude that He cannot exist.
 

The only way you could be 100% sure there is no God is if you yourself were "all-knowing"... and you're not.

Choose your response:

OK, maybe I can't really be 100%sure that God does not exist

I still think everything evolved, and that's where it came from.  Science supports this position.

I disagree, as an atheist, I can be sure there is no God without being all-knowing.