A Good Place To Start
The word most often translated "eternal" or "everlasting" in relation to punishment is aionios, a word whose meaning in both classical and biblical Greek is far more nuanced than our English translations suggest. G. Campbell Morgan, Marvin Richardson Vincent, Thomas Talbott, and David Konstan each shed important light on what this word actually meant to its original readers.
“Let me say to Bible students that we must be very careful how we use the word ‘eternity’. We have fallen into great error in our constant use of that word. There is no word in the whole Book of God corresponding with our ‘eternal’, which...
“Aion, transliterated aeon, is a period of longer or shorter duration, having a beginning and an end, and complete in itself. The adjective aionious in like manner carries the idea of time. Neither the noun nor the adjective, in...
“Eternal life, then, is not merely life that comes from God; it is also the mode of living associated with the age to come. And similarly for eternal punishment: It is not merely punishment that comes from God; it is also the form of...
“…aionios does not mean ‘eternal’; it acquires this meaning only when it refers to God, and only because the notion of eternity was included in the conception of God. In particular when it is associated with life or punishment, in the...