Christianity in the roman empire.

[Jona Lendering]

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.

Yes, and we might also recall the Hymn to Zeus by Cleanthes, another Stoic. I think there may be other examples as well; I recall Aristotle's idea about one transcedental God, but I do not remember the details. Yet, in my initial posting I was thinking of Neoplatonism, which was what Augustine and his companions used to present Christianity to the Pagans as a logical faith.