Did ancient Greek religion ever become monotheistic? Unlike modern monotheistic religions that only accept one, omnipotent and omniscient god, ancient Greek religion has a pantheon of fallible gods. Yet it was also well developed, where the pantheon has hierarchy (Zeus, king of gods) and lineage. Was there ever a time and place where the religion evolved into a monotheistic form? That is, only accepting one god, rejecting the others as false ones or manifestations of the one true god. For example, ancient Egyptian religion briefly gave rise to Atenism, the offshoot religion that states the sun-disc _Aten_ was the only god. Did a similar thing happen to Greek religion? If so, which god became the one? Or was it an amalgam?

No, at least not that we know of. If there was a monotheistic cult in ancient Greece, it certainly wasn't as popular as Atenism. There are traces of monotheistic thought in Platonism (e.g. the Euthyphro dilemma), but that's more about philosophy than religion.

There's one theory, put forth by Elizabeth Kessler in _Dionysian Monotheism in Nea Paphos_ that "two monotheistic religions, Dionysian and Christian, existed contemporaneously in Nea Paphos during the 4th century C.E.". The only evidence cited, however, is a mosaic where Dionysus is the central figure, so I'd take this with a grain of salt.