I read this when it came out in the 70s. 666 is based on the interpretation of a genre of writing called Apocalyptic, a highly-symbolic mode of literature found in the Jewish and Christian Scriptures. I was in conservative religion, like the author, of a literalist-biblical orientation. It was scary to me, and it spoke of a future I had been taught was to happen. So, 666 is a work of fiction to portray a future that for many, if not most, conservative Christians is not fictional at all.
I came to disagree thoroughly with this kind of teaching. This appears a literalizing of literature that was highly disguised to speak to the faith communities in cultures where persecution from outsiders was a threat. So, Apocalyptic is code-language, so to speak. To read Apocalyptic literature as telling of persons being raptured bodily into the sky, or disappearing suddenly, or having numbers on the head, or being thrown into a lake of fire, or Jesus coming out of the sky riding a white horse is a disservice to the genre and the community or writer(s) that created this literature.