In Christian mortalism, what sets apart people who have lived from people who didn't live?

Once a life is conceived a cloud back up account (God’s memory)is opened for that individual.

When that soul expires the account is inactive but fully accessible by the cloud administrator.(God)

At a time of His choosing God will resurrect that soul(life) and download the backed up files from the cloud account into that new life(soul).

In order for there to be an account in the cloud there has to have been life.

God does not have cloud accounts set up before life is conceived and then assign that account to a new soul(life). God does not maintain an inventory of souls in a heavenly warehouse to ship immediately to the womb of a newly pregnant woman.

There is no separating life from soul. For this reason I can’t fathom what you mean when you speak of a person who never lived.

God’s being able to foreknow everything does not mean he chooses to do so. Instead he uses selective foreknowledge.

My answer comes with a caveat:

I have struggled to grasp the meaning of this question, and have been confused by that Wikipedia article on what it calls “Christian Mortalism”. I have therefore decided to put that to one side and simply address the following questions:

What sets apart people who have lived from people who didn't live?

My understanding is that people who have lived were born of a woman and received the “breath of life”. On the other hand, “people who have not lived” were never born with a human body. Perhaps that would apply to a foetus that died while in the womb – a still-birth – and therefore never received “the breath of life”.

This is the only difference I can think of that would set apart a person who lived from one who did not live. From the moment fertilization takes place, the child’s genetic makeup is already complete. The only thing the embryo needs to become a fully-functioning human being is the time to grow and develop. Human life begins at conception and the unborn child is already known by God:

My frame was not hidden from you when I was made in the secret place [the womb]. When I was woven together in the depths of the earth, your eyes saw my unformed body. All the days ordained for me were written in your book before one of them came to be (Psalm 139:15-16; Jeremiah 1:5).

The foreknowledge of God is far more than His ability to “see the future”; His foreknowledge is a true “knowing” of what will come to pass, based on His free choice. He decrees what will come to pass. In other words, foreknowledge is not just intellectual; it is personal and relational. Foreknowledge is equivalent to foreordination in that God ordains, or orders, all that will be. https://www.gotquestions.org/foreknowledge.html

How could God not grant resurrection to a person never having existed?

Working on the basis that you are enquiring about an unborn child that never had an opportunity to draw its first breath, are you really asking whether an unborn child will be resurrected by God? The Bible does not say, but we know that God is merciful as well as just; He is a God of goodness and mercy; He is gracious in all His works. While the Bible does not teach universal salvation, we know that Jesus died “for the sins of the whole world” (1 John 2:2).

Will the body/soul of an unborn child be resurrected?

Only God knows, but we can be confident that His steadfast love endures forever (Psalm136) and He is righteous in all His ways (Psalm 145:17).

For all Christians who understand that the soul continues to exist after the body dies, they can draw comfort from knowing that the soul will be clothed in a resurrection body at God’s appointed time. Our God knows the beginning from the end – He is Alpha and Omega – He is the Sovereign Ruler.

P.S. If I have misunderstood your question, please forgive me.

Disbelief in an immortal soul is one thing (and you have given examples of some such groups) but confusion may arise with the closely related belief of 'soul sleep'. It is also necessary to clear up what the word 'soul' means for those groups that disbelieve in an immortal soul, and those groups that believe the soul 'sleeps' in death until the time of the resurrection.

The Christian religion has always taken seriously what the pre-Christian saints said about a future resurrection of the dead. Here are three examples:

"I know that my redeemer liveth, and that he shall stand at the latter day upon the earth: And though after my skin worms destroy this body, yet in my flesh shall I see God; whom I shall see for myself, and not another." Job 19:25-27 A.V.

"Thy dead men shall live, together with my dead body shall they arise. Awake and sing, ye that dwell in dust: for thy dew is as the dew of herbs, and the earth shall cast out the dead." Isaiah 26:19 A.V.

"And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt." Daniel 12:2 A.V.

Everyone knows that the ancient Hebrew scriptures did not contain a developed doctrine of the resurrection, but those three texts alone show there was belief of a coming day when "the earth shall cast out the dead" who will live again and be judged.

Jesus' words explain everything. He was totally clear that it would be at the sound of his voice that the dead would come forth. It wouldn't be the voice of any woman's husband, or the voice of a prophet of old that would bring other people back to life. No. It is because Jesus has the keys of death and Hades, and has conquered the grave, that he alone has the God-given authority to raise all the dead. And it is, indeed, ALL the dead who must arise on the Day of Resurrection and Judgment, because that's what Jesus has said.

"Marvel not at this, for the hour is coming, in the which all that are in the graves shall hear his voice, and shall come forth; they that have done good, unto the resurrection of life, and they that have done evil, unto the resurrection of damnation." John 5:28-29 A.V.

"Jesus said unto her, I am the resurrection and the life; he that believeth in me, though he were dead, yet shall he live. And whosoever liveth and believeth in me shall never die." John 11:25-26 A.V.

"I am he that liveth, and was dead; and behold, I am alive for evermore. Amen; and have the keys of hell and of death." Revelation 1:18 A.V.

God knows every living soul, and every dead soul, and all those who will yet become living souls before Christ suddenly returns to usher in the Day of Resurrection and Judgment. On that aweful day shall all the dead, "small and great stand before God; and the books were opened... And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them; and they were judged every man according to their works. And death and hell were cast into the lake of fire. This is the second death. And whoever was not found written in the book of life was cast into the lake of fire." (Revelation 20:12-15 A.V.)

That leaves nobody out of the resurrection. They are all given resurrection bodies in order to stand before God, and to go on to experience God's judgment of them - either in glory with Christ eternally, or in torments in the eternally burning lake of fire. They are not disembodied spirits once their spirits have been united to their new resurrection bodies. Those are bodies made fit for eternity, either in glory or in torments (see Revelation 14:9-11.)

For as long as there is any thought about "potential people" being mixed in with now physically dead people, the matter is confused. Those who have died do not cease to exist (as with soul annihilationist ideas). They do await a new resurrected body to be united to their spiritual part, in order to be judged and then go to what God has decreed for their eternal state. That is as clearly as I can put the mainstream Christian teaching.

All souls are mine

Ezekiel 18:4

Life begins at conception when a child is formed in the mother's womb. The soul lives inside the mother's womb and even if the soul never gers to breathe air, the accounts for that soul exist with God. God knows every single person that has ever lived or who will ever live.

The Dead shall hear the voice of the son of God and they shall awake, some to everlasting life and some to everlasting anguish and torment

The soul dies with the body and awaits the resurrection, it's the inner man or the spirit that receives a spoiler from God on the eternal destiny of that man when the spirit is received either in paradise like Lazarus or in Hades like the rich man.