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We can leave for the literary critics the question of whether Mark Twain was making a theological statement about the devil, showing what a kind person his mother was (but religiously orthodox; the quotation follows a bit where she's quoted as asserting the devil is in hell), or needling his readers with the possibility that he's doing both. Clemens and mom Jane lived on the other side of Christian Universalism becoming an acknowledged if still scorned belief. But in fact, Christian theologians and then modern scholars have been preoccupied with the question of "who thinks Satan can be/is saved" for most of Christian history.

Most of the efforts in this field have focused on patristics (early Church), especially the Greek Fathers. Efforts that go back to the early (slash early medieval) Church. In 543 and then maybe 553, a local synod and then the Second Council of Constantinople condemned a teaching they labeled apokatastasis, which they associated especially with the works of very important theologian Origen of Alexandria (d. 254). Absent any more technical context, like astronomy or theology, the Greek means return or restoration to an earlier status or the original status. To the heresy-smashing synod and council delegates, apokatasis referred specifically to the return of all souls to God and the dissolution of all things into God. Also the textual tradition of the actual words is questionable, later centuries would believe that the Council, acting for Justinian, condemned Origen as follows:

If anyone shall say that all reasonable beings will one day be united in one, when the hypostases as well as the numbers and the bodies shall have disappeared [i.e. the persons of the Trinity, particularly Christ]...let him be anathema.

If anyone shall say that the life of the spirits (νοῶν) shall be like to the life which was in the beginning while as yet the spirits had not come down or fallen, so that the end and the beginning shall be alike, and that the end shall be the true measure of the beginning: let him be anathema.

A second set of anathemas, attributed to Justinian himself, makes it abundantly clear that the belief being anathematized under this deployment of the term apokatastasis is the reunion of all things to God, includes demons and the devil.

So medieval theologians in the Latin West inherited these anathemas, understood as coming from an ecumenical council/one of the most important sources of canon law, and the dogma they represented: no universal reconciliation of human souls; no reconciliation of demons and the worst demon. This is absolutely bedrock medieval theology.

Even the most neo-Platonist and mystically minded of the major Latin theologians, who write passionately of a universe where all things proceed from God and all things return to God, explicitly exclude the dissolution or annihilation or assimilation. Hell is eternal; the devil is there and so are the damned.

The Church guarded this doctrine zealously. In a 1231 antiheretical campaign, a woman named Lucardis was burned at the stake-despite her "saintly life"!-for the treacherous crime of "bewail[ing] with dreadful laments the unjust banishment of Lucifer, whom she wished again restored to heaven." I repeat: her crime was wanting God to be "too" merciful. Medieval preachers sometimes found hell a rough doctrine for their congregations to accept--how God could condemn so many and save so few, especially if God was supposedly merciful. But this only doubled the motivation of, for example, Parisian bishop Guillaume of Auvergne to drive home the message of eternal damnation to his listeners.

Or take Origen, whose mystical theology pops up in all sorts of places in the Middle Ages despite the anathemas. Holy women gifted with divine visions, including visions of the afterlife, frequently fielded questions on whether Origen was in heaven or hell. Gertrude of Helfta in the late 13th/early 14th century skillfully dodged the question. As valued as Origen and his ideas were, they couldn't set aside the problem that the Church had declared him a believer in the restitution of all things to God.

As far as the Church was concerned, that could have been it. Even with free will and salvation such central controversies in the theologies of the Reformation era, no even semi-organized group supported any kind of universal salvation. But the rise of Universalism out of 18th century natural theology and pietism triggered the first reevaluation of earlier theologians. Looking for forefathers, they claimed Origen with zeal and added fellow Greek Fathers Clement of Alexandria and Gregory of Nyssa to the mix. Even early medieval theologian John Scotus Eriugena (not John Duns Scotus of Immaculate Conception fame), who in his era was an island of Greek language and textual knowledge in a Latin western sea, was sometimes tapped as alluding to apokatastasis in the technical sense.

Then modern scholarship and scholarly methods came along and basically blew everything up.