Just came across this very thorough study of the writings of the Early Church Fathers relating to Conditionalism and Perpetual Conscious Torment: The Doctrine of Immortality in the Early Church, by Dr. John H. Roller (free download).
Roller goes through all the uninspired writings of the early church fathers to determine who was and wasn’t a Conditionalist or a believer that even the damned have immortal souls.
The Apostolic Fathers are the early church fathers early enough to have known the apostles (whether or not they actually met an apostle). Regarding their preserved writings, Roller concludes,
Furthermore, since we have now completed our study of the Apostolic Fathers, and found none of them to be Naturalists, I must agree with Dr. James K. Brandyberry’s conclusion that, “the teaching of innate immortality is absent from the Apostolic Fathers, those Christian writers who lived nearest to or whose lives partly paralleled the last of the apostles.”
Regarding the next oldest group of writers, which he calls the Sub-Apostolic Fathers — taking his study to nearly 200 AD, he concludes,
There is no question but that Irenaeus of Lyons was a “champion” of Conditionalism. We see, then, that the age of the Sub-Apostolic Fathers comes to its conclusion at a point in time prior to which only one Christian writer (Athenagoras) has espoused the doctrine of Natural Immortality, all the others (of whom we have studied a total of seventeen) having held, more or less demonstrably, to Conditionalism.
He later concludes that before Clement of Alexandria (end of 2nd/early 3rd Centuries), 18 authors were Conditionalists; only one was not. After Clement of Alexandria (that is, during the 3rd Century), the numbers flip to two Conditionalists and 8 not.
Obviously, something big happened around 200 AD to shift Christian thinking. Roller does not detail the history, but other sources suggest very credibly that it was the philosophy of Neo-Platonism, a school of thought that came to be the dominant worldview of the Roman Empire around this time. As a worldview, it was taken by most as so obviously true, that its merits weren’t even debated.
Origen and Augustine were Neo-Platonists, and very influential on Christian thinking. The central teaching of Neo-Platonists was the origin of all things in the One — which would easily be blended into Christianity. Neo-Platonists insisted, along with Plato, on the immortality of the soul — so that the damned must live forever, resulting in gehenna becoming Dante’s Inferno. In fact, in a culture defined by this philosophy, it would be rare to even ask the question.
Just as it is today.