When we come to the Patristics, we find that the earliest writers go in three directions. Origen taught a form of universalism. Tertullian taught everlasting torment. But perhaps the earliest of the preserved, uninspired writings teaches annihilation. The Didache 16:13-17 says —
Then shall the creation of men come into the fire of trial, and many shall be made to stumble and shall perish; but those who endure in their faith shall be saved from under the curse itself. And then shall appear the signs of the truth: first, the sign of an outspreading in heaven, then the sign of the sound of the trumpet. And third, the resurrection of the dead — yet not of all, but as it is said: “The Lord shall come and all His saints with Him.” Then shall the world see the Lord coming upon the clouds of heaven.
This work dating from the late First Century to early Second Century, teaches that only the saved will rise again.
In A Treatise on the Soul, Tertullian (writing from 197 – 220 or so) argues that souls are immortal. After all, he assumes, the only alternative would be that the soul dies when the body dies. Because souls necessarily last forever, and because we’re taught the damned will suffer torment in the next age, surely this torment also lasts forever.
By the 5th Century, eternal torment was increasingly the orthodox teaching. Augustine wrote at a time when many still taught annihilation (as he says himself), but he strongly advocated for eternal torment. In so arguing, he was highly influenced by Greek philosophy, particularly Platonism. In Platonic thought, the soul is inherently immortal.
Augustine was hugely influential in Catholicism and Protestantism, as he managed to merge much of the Grecian philosophy of the day with Christian principles. His writings dominated Christian thought in many areas for centuries, including his arguments for an everlasting hell. And his writings remain highly influential.
Indeed, even modern Christians often assume that souls pre-exist birth and are assigned to babies as they are born or conceived. But this is Platonic. Jewish and New Testament thought does not draw such a distinction between body and soul.
You see, the false assumption that we live forever as disembodied souls has led to two errors — that the saved will live in heaven in disembodied bliss and that the lost will live in hell in disembodied agony. But neither view is Biblical. It’s just that the ideas are so old and so deeply rooted in our psyches that few of us have even thought to question them.