Christianity in the roman empire.

[Epictetus]

Pagan intellectuals had, since the second century, developed ideas that behind the multitudes of divinities was one single God,

Perhaps even earlier than that, if Diogenes Laertius is to be believed. He says that the father of Stoicism, Zeno, taught this in the third century BC.

They [Zeno and Chrysippus] also say that God is an animal immortal, rational, perfect, and intellectual in his happiness, unsusceptible of any kind of evil, having a foreknowledge of the world and of all that is in the world; however, that he has not the figure of a man; and that he is the creator of the universe, and as it were, the Father of all things in common, and that a portion of him pervades everything, which is called by different names, according to its powers; for they call him Dia as being the person (di hon) everything is, and Zeus, inasmuch as he is the cause of life, (tou Z?in), or because he pervades life. And Athene, with reference to the extension of his dominant power over the aether (eis aithera). And Hera, on account of his extension through the air (eis aera). And Hephaistos, on account of his pervading fire, which is the chief instrument of art; and Poseidon, as pervading moisture, and Demeter, as pervading the earth (G?). And in the same way, regarding some other of his peculiar attributes, they have given him other names.