I was not born yesterday. I have seen decent people in a school I went to doubt Gods existence, and then gradually evolve into Satanists.
A very famous atheist Aron-Ra recently admitted hes been a closet Satanist for awhile now. (no kidding, I knew that for years, I mean just look at the guy, he looks like a warlock)
Im sure you have seen it yourself, when kids at your school first deny God, then turn to witchcraft.
Anyways, it seems that when people throw out God from their life, they inevitably replace Him with another, lesser god—namely themselves, at least, at first.
When the Serpent deceived Eve in the garden, his argument that won Eve over to the dark side was convincing her that 'she would be like God', which is precisely the Atheistic Evolutionary goal for the Ascent of Mankind'.
Now, what Adam and Eve didnt realize was that, by listening to the Serpent, they were not simply denying God and 'becoming their own god', but rather giving credence to the voice of the devil, replacing God's words with Satan's words, which they obeyed.
Atheists make themselves God's enemy, and the enemy of their enemy is their friend. In this case, Satan is their friend. And similarly, as you said, they become useful idiots, but for Satan to exploit.
Atheism is but a stepping stone to Satanism. Facts.