Ambrosius Autpertus (An 8th century Bennedictine monk, abbot, and Theologian who wrote commentaries on Holy Scripture.)
Rev 20:10 “And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are: and they shall be tormented day and night for ever and ever (Latin saecula saeculorum)." But when it says, "they shall be tormented day and night for ever and ever (Latin: saecula saeculorum)," it is not to be taken in such a way that no sentence of God’s mercy is left at all: but because very long times (Latin: saecula saeculorum) are appointed for punishments, which human frailty cannot measure. It can happen that even the evil spirits themselves, who are now obstinate in their wickedness, after very long times are freed by God’s mercy, when they put off the nature which they corrupted, and are reformed to that for which they were created. But if this is so, it does not detract from divine justice, because they will have paid for a very long time what they deserved.
Source: Ambrosius Autpertus--Commentary on the Apocalypse (c. 784 AD), Book 10, ch. 36. Patrologia Latina 89:1391.
Agobard of Lyon (9th century Archbishop of Lyon)
"Nor should one despair of any creature made by God, that it could not return to a better state, provided it itself wills."
Source: Liber de Divina Psalmodia, Chapter 20