1) The vastness of the universe. Theists would have you believe it was all created by God. Inside 6 days. Now that we know that the universe is incomprehensibly large, and continuing to expand, encompassing billions upon billions of stars, almost certainly supporting other forms of life, what exactly is so special about a bunch of apes sitting on a rock?
2) Evolution. The evidence is in, and it suggests in the strongest possible terms that all life on earth evolved by the process of natural selection for fitness, from small populations of tiny, single-celled organisms to the vast range of life we observe on our planet today. This refutes the notion that all life was created by God, again within a couple of days.
3) Original sin. The whole of Christianity rests on the idea that humans are intrinsically sinful, and that this is because a woman, created from the rib of a man who was himself created from dust, was told by a talking snake to eat a magic apple which made her realise that her nakedness was disgusting. The whole notion is ridiculous, especially in light of our knowledge of evolution, and renders utterly pointless the other central tenet of Christianity…
4) The story of Jesus. First, we are expected to believe that Jesus had no human father, contradicting all we know about biology and human reproduction. Then we are given four accounts of his life that disagree with each other on countless details. Finally, we are told that he came back to life two days after dying of crucifixion, contradicting more biology, and then became one with God. And all this was to absolve humanity of the sin of a mythical woman eating a mythical magic apple. Leaving aside the question of whether he existed or not in the first place, the story of Jesus beggars belief.
5) The lack of supporting evidence. As scientific inquiry has shone light on our relationship to other life on this planet, as we have peered into the mysteries of the vast cosmos, as we have discerned the existence of sub-atomic particles that bind the universe together, so the flimsy “evidence” for God has disappeared, leaving behind only the plausible, naturalistic theories of existence. The ignorance that allowed religions to posit magical supreme beings as an explanation for everything has been banished, never to return, and we need bother ourselves with superstitions and fantasy no longer. The universe is amazing enough standing alone.