Why do you not believe in the concept of a God?

[Belacqua]

But I'm interested in what you said about Zeus being a fallen angel. It seems like that is some kind of cobbled-together ad hoc belief.

John can answer this in a more personal way, I'm sure. But I can supply some background.

The idea that the Greek/Roman gods were fallen angels is a very old one, probably from Jewish thought predating Christianity. The Christians took it up, of course. I don't know if it qualifies as ad hoc, but it was a kind of obvious transitional idea.  

For Jews their God was the best or most powerful. For Christians, their God was fundamentally unlike the Greek gods, being more like the Neoplatonic One, or the Form of the Good, or Actus Purus. By definition there can only be one of these, so a pantheon of gods doesn't make sense. They were comfortable, though, with the idea of beings intermediate between people and God, including angels. So back when belief in the Greek/Roman pantheon was still popular, the monotheists often didn't argue that those gods weren't real, but that they were intermediate, and not as powerful as had been previously thought. Some called them daemons -- but remember that for Greeks daemons weren't necessarily bad. Others associated them with the fallen angels who became evil daemons (demons in the modern sense) after the Fall. 

I've never been able to find a single source which originated this idea. It seems to have been pretty widespread. I first learned about it from manuscript illuminations which showed prophets or saints walking down the street and, through their superior virtue, causing Greek statues to fall off their columns. 

The idea of the Fallen Angels is only hinted at in the Bible, and is based mostly in the Enochian pseudepigrapha. (Writings attributed to the prophet Enoch, which probably weren't really written by him.) As you know, Hebrew scripture tends to deal more with this-world, political problems than Christian writings. Their Messiah isn't magic, but a political leader who will save the nation of Israel. Their prophets, even when having wild visions, are mostly either hoping for a renewal of the nation or are scolding their fellow Jews, whom they think have fallen into evil ways. So right from the beginning the Enochian writings were interpreted allegorically. The Fallen Angels were seen as, among other things, Jewish leaders or rabbis who had fallen away from proper behavior. 

Probably the key point is that, for strict Jews, the most worrying temptation at that time was Hellenization of Hebrew culture. They saw this as a dilution away from strict Torah practices. So it made sense to depict the bad influences, and people who had fallen away, as figures in the Greek pantheon. Saying that Zeus is a fallen angel was a way of expressing that Greek influences cause good Jews to fall away from their truth. 

John is the first person I've encountered who holds this belief today.