"If a Christian, say, wants to prove to a Muslim that God exists, the Christian won’t have to strain her mental faculties, because the Muslim is predisposed to accepting that conclusion. This would be like pointing out to someone that the grass is green."

A better metaphor would be that Columbus had to believe that the Earth is a sphere and going West would take him to India. I think that this also highlights the crux of the problem -- while theists assume that there is God, even if they find some hints they tend to interpret them falsely. Contrary to that, atheists dismiss the notion of God completely and thus continue to use the "Cape Route".

"Theism requires some similarity between humans and the universe’s ultimate source; as the Book of Genesis says, we were made in God’s image."

Well, there is such similarity -- "I am" aka consciousness-existence (also known as Brahman but also Tao, etc.; in Tanakh see Ex. 3:14). It's a literal 1:1 correspondence / identity. But since people identify "I" with human body and/or mind, theists have a hard time to shake off their incorrect and "idolatrous" images of God.