Around 1985/86 I read a book that was probably at least a few years old. It was a fantasy set in ancient times, where the pagan gods (it mostly focussed on the Greek gods) are real and interact with each other. I don't remember the whole plot, but towards the end, there is an invasion of their worlds by the angels of the Abrahamic god, who has become powerful enough to kick them out of their main realms, in parallel with real world events. He is depicted as having evolved to reach that point, there is some mention of him having eliminated the female figure in his own religion beforehand. The pagan gods & demigods go into lesser realms, but still exist. The book had a few b&w illustrations. It was very much of its time, the ideas about religion seem of a piece with a 70s counter-culture idea about hard done-by pagans. Any ideas what it could be?
After more searching, I've finally figured it out. The book is "The Gods Abide" by Thomas Burnett Swann. I would have read the original 1976 DAW edition, which does seem to have had some illustrations.