My understanding is that after the Great Flood and Noah, the people scattered thoughout the world and they carried the truth of the beginning of man (Adam and Eve), hell, a god, a savior, demons, etc. because the original truth stayed with them through oral history. It was passed down from generation to generation! That is why so many religions have similar truths! Even the Chinese have religious stories about the Great Flood!
Right! In addition some, such as the Greeks, came to realize (at least the more advanced ones) that there was one God, and came up with the concept of the Logos. To me that doesn’t say that Christianity was borrowing pagan ideas, but rather that God laid out the background for the Truth through his Son ahead of time amongst the cultures of that region.
Also, among the first to acknowledge Christ were the “Magi” (i.e. Zoroastrians) because the prophesies were in there religion true. That does not mean they had a true faith, but that God laid the groundwork for his greatest revelation, his glorious Son.