yfr's question about Twilight and why Criamon magi don't just drive themselves deep into it to gain benefits lead me to wonder something...
In earlier editions it was stated that magi who are lost in Final Twilight do not leave a ghost behind. Their spirit cannot be summoned by any magical means. Is this still the casein 5th edition? I don't see it mentioned in the core rules under Twilight.
Following from that, if magi do not leave a ghost behind and cannot be summoned, what is the state of a magus's immortal soul in Final Twilight? It suddenly occurs to me that, if Final Twilight actually represents spiritual oblivion (or even might represent spiritual oblivion) any good 13th century magus ought to be deeply terrified of it. The quesion of does a magus in Final Twilight go on to his final reward (or punishment) or does he suffer oblivion might even be a hotly debated topic in the Order. One's beliefs about it would serious impact the life of an older magus.
I'm no expert on this area, but I believe you are confounding two ideas. The summoned spirit (ghost) is not the soul. Yes, the Criamon still kill their primi so their ghosts are not lost due to twilight. Also, the ghosts of those buried in holy ground cannot be summoned, if I'm not misremembering. So that your ghost cannot be summoned does not seem to imply your soul has not gone to heaven.
It's one of those theological issues that shouldn't have an in-game answer. Just because magic can't summon something doesn't mean it doesn't exist.
That said, I would expect most 13th century thinkers to doubt that a mortal could avoid final judgement that way. At most, the magus in twilight would just remain outside of Heaven and Hell until Judgement Day comes.
Once I would have agreed with you, but RoP: Magic, Ancient Magic and The Church have all cast doubts on that for me. RoP: Magic has an extensive section on ghosts, including a sidebar raising the question of where they fit theologically, which I will not reprint in full but is sufficent to cause me to wonder if ghosts are simply souls in purgatory (certainly some ghosts think so and the text leaves it deliberately and expressly ambiguous). The Church states that Extreme Unction protects the body from magic for only three days. Ancient Magic states that only those who have recieved holy burial rites can be effected by Canaanite necromancy (the immunity to magic granted by Extreme Unction is a flaw in hermetic theory). Ancient Magic also notes that magi in Final Twilight are not "dead" for purposes of Canaanite necromancy.
Taken all together, I'm surprised that the state of the soul in Final Twilight has not already been a subject of discussion. If I were a 13th century magus, I would certainly be giving the matter some thought.
Ghosts are (often) summonable/affectable with Hermetic Magic - which has a rather large limit in that it cannot affect anyone's soul. So, I'm with Callen on this.
The game system leaves a lot of metaphysics unanswered, on purpose, but it is clear that there is nothing that Hermetic magic can do with/to souls.
Mind = mentem; mentem is used on/against ghosts, ergo ghosts are a sort of mind. (Please don't start arguing logic/semantics again, folks!) Now, whether a soul somewhere is generating these ghosts in some active way, or if they are merely like recordings is up to your saga. (Now I've just thought about how certain other games have 'outsiders' generating a body in the material world; destroying that body merely annoys the original outsider, and you have to go to their originating plane to actually 'kill' it properly - ghosts could be done in a similar way, being re-made by the soul - this way you can have a ghost that has reformed and now remembers how some magi 'killed' it...)
If you want to know about souls, consult a priest that you (or rather that your characters) trust...I don't think any version of doctrine covers twilight, so maybe you'll have to get a Jerbiton with the right connections to provoke an official ruling from the Pope/Convocation of Bishops/other religious body. What answers you get, and how useful those are, depend entirely on the game/the storyguide/the troupe you're playing with. Have fun.