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Origen
Plutarch
Maximus Tyrius
Aristotle

These names aren't just men who believed in a single God, they're names of men who made a broad study of ancient religions and concluded that all religions were paying homage to the same notion of a perfect unity being responsible for creating and ruling over all of existence. There are also plenty of examples of authors like Cudworth discovering the same thing in the 1600's, but I gave you examples that were closer to the source of the religions they were studying.