On The Patristics Project website there is a blog post entitled "Patristic Universalism", which provides quotes from twelve of the ancient Christian writers popularly known as the Church Fathers, regarding how they, among others, "either believed in the universal restoration of all things, or stated things with Universalist implications." The post goes into some definitions of universalism although this, I think, is best elucidated by references in the post's section "The Universalist Majority," which says:
In the work “De Asceticis,” St Basil allegedly states, “The mass of men say there is to be an end to punishment and to those who are punished.” St Augustine concurs by allegedly saying, “There are very many in our day, who though not denying the Holy Scriptures, do not believe in endless torments.”
The last section of the post is titled "The Divine Liturgy," towards the end of which its author offers the following (with my own emphasis [and footnote]).
Personally, I do not identify as a Universalist, so I do not want people to get the impression I am trying to sell it, as I am merely presenting the objective patristic data... I have my own reasons for disagreeing... I would probably be much more zealous in my rejection of universalism, if it were not for the liturgy making a hypocrite out of me. I say evil people who reject God will not be saved, yet I pray for the salvation of those same evil people who reject God even after they die. I have heard that there are even monks on Mount Athos who pray for the devil. Is it any wonder why Fr. Sergius Bulgakov✭ arrived at such a place? Like Moses, it is the heart of love that persistently seeks to change God’s mind (so to speak) to actually give people precisely what they don’t deserve (Exo 32:14). This Mosaic prayer is constantly uttered unto God by the saints on behalf of all humanity.
The two Church Fathers who are most famous (at least in the modern era) for having been universalists, namely Origen, and Gregory of Nyssa, are incidentally also the ones known for having espoused the idea that not only all humanity was or would be saved by/through Christ, but that the devil and evil spirits/entities/demons in general, as logikoi, i.e. rational beings, are likewise beneficiaries of this aspect of God's grace.
That being as radical as it appears to be when compared to the majority of Christian tradition over the course of history, I had never before encountered the yet more radical idea of praying for the devil, presumably for his salvation(?). (I don't see what other reason there could be to do this, at least as far as the most mainstream Christian views of the devil and his role are concerned.)
As far as I have read, neither Origen nor Gregory mention such an idea. In the modern era there are both children and adults who have given the question serious enough thought to ask authority figures about it, based on the commandments and doctrine of loving and praying for one's enemies, as I discovered on Billy Graham's website, the Catholic Answers website and the Answers in Genesis website.
I tried to contact the cited blogger for more information on my Question but have received no response, and my research on it turned up just one hit, from the St Andrew Greek Orthodox Church in South Bend, IN, USA. It has a website with a page entitled "Satan and Demons," written by the parish's priest, the Rev. Fr. George D. Konstantopoulos, Protopresbyter, who says:
It is said that Saint Paisios used to pray for the Devil, until one day Saint Paisios saw him laughing in a distance from his cell and I think ridiculing the Geronda (Elder). From that day Saint Paisios stopped praying for the repentance of the Devil, since he understood that it was futile [emphasis in original].
Paisios of Mt Athos was a Greek Orthodox ascetic (1924–1994) who spent a good chunk of his life on Mt Athos. This is the only other reference that I have found to a monk from that mountain praying for the devil, and it is the only place I have seen this story told about Paisios.
Similarly to the conclusion arrived at by the paragraph which tells the story, the aforementioned websites pretty much roundly denounce the very thought of praying for the devil, for essentially the same reason. It seems to be across the board regardless of whether the specific Christian tradition in view is Eastern Orthodox, Roman Catholic, or Protestant, none of which is surprising.
But how obscure is this idea or practice, considering that, as early as Origen (200s AD), and Gregory of Nyssa (300s AD), there were Christians who did not dismiss out of hand the concept that the devil would or could be saved (along with all of humanity)?
Is Paisios the only Mt Athos monk who ever prayed for the devil? Are there other Christians elsewhere who have gone beyond mere inquisitiveness regarding this (see the websites above) and thought to do the same in an overt manner or an "official" capacity?
✭ Sergius/Sergey Bulgakov (1871-1944) was a Russian Orthodox theologian, philosopher and economist who apparently believed in universal reconciliation or was quite hopeful about it being the destiny of humankind.