I have read that many of Philos opinions mirrored what Paul supposedly taught. But back to your question about Paul miss using scripture. If there was any scriptures it looks ti me they were Gnostics writtings. I don’t think any other scriptures were much around at the time since many scholars believe the gospels came after many of the writings attributed to Paul.
I believe the whole Christians concept was a mix of teachings from the older Osiris cult and other Gnostics ideas thrown in. It’s been written that many of the ideas in the New Testament were only allegory’s that later became taught as real stories and characters to convince the crowds to except Christianity.