The Atheist is a document in Outlast 2.
This document is hidden next to a dead body in a small lit up cavern that can be reached by going through a gap in the caves in Val's Rebirth. This gap is located on the opposite side of the ladder you reach just after escaping from two Heretics in a large subterranean lake.
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"Dearest Beloved.
Please don't run. Please read this. Please forgive me. I know I must look dog-ugly and beat over. I cannot talk to you straight because I've lost my tongue to another man's teeth in an act it pains me to think back on. I left the world for Papa Knoth when I was just a boy; and believed everything he told us for most of my life. But a man can only take so much abuse; can only swallow so many world's endings. It is hateful to try to believe God's silence is a sign of your own failings. Val tempted me with his twisted up gospel and I believed. And I felt something move upon me more powerful and real than any holy spirit. Even those nightmares of what we done that plague us sleeping and wakeful both were silenced. I reveled in sin like a diseased animal, and it gave me blessed peace. But the pleasures in rutting and blood can only go so far and the nightmares are finding their way back in. I can't find no solace in God and if there's a Devil he's God's work, too, and I can only hope ain't none of it is real. There's despair in this, and I find I miss you. I hope it is love and not cowardice. I hope you can forgive me. My worst sin was letting you go. I'll love you as long as you let me.
Please forgive me."