The doctrine of original sin

The doctrine of Original Sin in Christianity is the cornerstone of a number of central beliefs, including the godship of Jesus and the linkage between the crucifixion and salvation.

Amazingly, the core belief of Christianity, which is that Jesus died as a sacrifice for our sinful nature and was resurrected to show that through his blood the sin of Adam (Original Sin) has been forgiven by God, seems to be based on a theory developed by leaders of the early Catholic Church, rather than on scripture. Jesus himself doesn't seem to have talked about Original Sin.

The theory based largely on the text contained in some of the epistles of the apostle Paul, including...

"Therefore as sin came into the world through ONE man" (1 Corinthians 5:21)

and...

"For as by a man came death (sin), by a man also has come the resurrection of the dead. For as in Adam all die, so also in Christ shall all be made Alive." (1 Corinthians 15:22)

Men such as Tertullian (160-220 AD) and Cyprian (200-258 AD) first formulated the doctrine in their writings. The "theory" of Original Sin, which was later popularized by Origen, Augustine and John Calvin, became a doctrine of the Catholic Church in the 5th century AD.

The question is... "Why was the theory of Original Sin accepted by the early Church if it was not based directly on scripture?

If you study the Bible closely, you see that there are many verses in the Old Testament that contradict Original Sin, including Ezekiel 18:20, which reads...

"The soul who sins is the one who will die. The son will not share the guilt of the father, nor will the father share the guilt of the son. The righteousness of the righteous man will be credited to him, and the wickedness of the wicked will be charged against him".

What could have accounted for the need to adopt this un-Biblical doctrine?