I just started reading “Early Christian Doctrines” by JND Kelly. I bought it about 6 years ago while researching Orthodoxy, but only now have I gotten around to reading it….
So far it’s fascinating. It starts off with the religious and philosophical background of the centuries surrounding the birth of Christianity. Judaism was forming concepts of hypostases of God, of God’s traits becoming personified: Wisdom, glory, etc.
People were getting tired of the usual paganism and beginning to see the various gods as different manifestations of one God: monotheism. The “mystery religions” claimed to bring you to unity with God.
There was an afterlife and ghosts, but it was a very dark, depressing existence; ghosts were feared because they were jealous of the living. But there was also a concept of rewards and punishments.
In philosophy, there were concepts of the logos (before it was defined as Christ), fate and Providence, and a concept of ethereal fire which sounds very much like energies and the River of Fire. There is also the division of man into such parts as soul, body, intelligence, and a discussion of material vs. immaterial.
There are, of course, two different ways people seem to take this information: 1) As evidence that Christianity was a product of its age. 2) As evidence that, as the Church Fathers would have it, God was revealing Himself even before Christianity and outside of Judaism, through the pagan philosophers.