God warned the Israelites not to "learn the way of the nations at all, and do not be struck with terror even at the signs of the heavens, because the nations are struck with terror at them. For the customs of the peoples are just an exhalation." (Jeremiah 10:2,3)
Far from borrowing from pagan ideas, Jehovah commanded his people "to remove the gods that your forefathers served on the other side of the River and in Egypt, and serve Jehovah." (Joshua 24:14) When some Israelites tried to combine Jehovah's worship with Egyptian calf worship, Jehovah swiftly indicated his great anger at this. (Exodus 32:7-10)
Repeatedly throughout their history, the nation of Israel turned to pagan practices, and God repeatedly punished them for this.
I believe the early Christians followed closely the teachings and example of Jesus Christ. It was after the death of the apostles that a great apostasy from true worship occurred, just as warned about by Jesus and the apostles. (Acts 20:29,30; 2 Thessalonians 2:3-5) And it was then that an apostate form of "chrisianity" fused with pagan ideas from Greek thinking and philosophy. This unholy alliance, I believe, led to such doctrines as the trinity, immortality of the soul, and hellfire, none of which are taught in the Bible.