A nice and simple way I like to explain the atheist position to those who don't understand or object, goes like this. Think of a Christian, he doesn't believe in Mohammad's revelation by the angel Gabriel. He doesn't believe in the Koran which was the written word of Mohammad's revelation. This is the central belief that all Muslims must believe in, that Mohammad was a prophet and revealed to mankind God's message. Take the Muslim for a moment. He doesn't believe Jesus was the son of God, died on the cross, and was resurrected three days later. He thinks that the Jews substituted Jesus with another body and that Jesus was buried somewhere. This is a denial of the central story of Christianity that all Christians must believe.
They each think the others' central stories for which the religions are founded on are made up bullshit. What we as atheists simply do is go one step further and take the position that religious people each have about all the other religions in the world, that is that they are false. We simply believe that all of religions fantastic miracles and revelations are man-made and not true. From the Muslim and Christian perspectives, the others' miracles aren't founded on any truth, despite the billions believing them. They know that there is mass delusion going on in the other camp, oh but not theirs. They have the truth in their book. It seems more likely that if they deeply believe the other is false it seems more likely that they are both false, since them both being true would contradict each other.