Actually, you can say with certainty that God, an omnipotent, omniscient, all-powerful being, doesn't exist. Why? Because there is, and never has been, any evidence that he does. The basis of belief should be evidence, not hypothesis.
It is more logical to choose a particular God because at least you have historical evidence of Muhammed or Jesus existing. You also have a bunch of people who have had visions and what not attest to such. Whereas the agnostic first rejects such visions, then proceeds to develop one of his own. This is infinitely more insane, as delusion is determined by consensus.