However, Phelps "ministry" seems to exist, simply to shock. He's like the one dimensional villain who is evil for the sake of being evil.
Since his public persona seems like a fictional character, I assume he is one.
www.atheistnexus.org/page/nate-phelps-2009-aa-speech
trigger warning/tl;dr: Phelps family is every bit as horrifically abusive as you'd imagine.
That's how I see the Pope or your typical small town preacher, yet I believe their sincerity. I'm not sure why you get to sit there and decide who is sincere and insincere without any proof. They are Christians, they follow scripture, they have opinions and audience. They're not con men, they are simply misguided and in IMHO evil because its impossible to have a non-evil religion. At the end of the day, believing woo that justifies your biases is evil.
Phelps is just a Christian. Accept it. Move on.
I don't think he's an atheist, though. I know plenty of Christians who are almost as crazy and definitely not acting. I think he's just a fundamentalist who's smart enough to take advantage of adding a little on the top; a sort of larger-than-life "media personality."