I believe that we, all of us, existed as pre-mortal spirits; and that we chose to be incarnated into this mortal life (from which we will be resurrected). This doctrine, I got from Mormonism, where it is a key part of the revelations from Joseph Smith; but I believe it because of direct intuitive experience... because I 'remember' that it was so.
(That is the proper - although not sole - reason for believing any key Christian doctrine; indeed until one has such a personal revelatory conviction, any belief tends to be 'just a theory', and thereby feeble or fear-based.)
However, as Terryl Givens makes clear in his scholarly book When Souls Had Wings (OUP 2009) - this idea of pre-mortal existence is Biblical, and was a belief of several of the early church Fathers, before it was 'oulawed' as heretical by later church councils.
Nonetheless, the belief in a pre-mortal spirit existence has remained a minority thread within Christianity throughout its entire history - and could perhaps be regarded as heterodox rather than heretical.
Anyway, having understood and assented to the reality of pre-mortal life; I have come to realise how much mainstream Christianity lost by excluding it and instead insisting that our souls were each created from nothing sometime between conception and birth.
For a start, the exact point at which the newly-created soul is joined with the body seems important, yet all arguments I have seen on the topic seem unconvincing, arbitrary.
Secondly, when the soul is newly-created from nothing, then there is no possibility of our having agreed to incarnation - leading to the 'I didn't ask to be born' notion, so common in modernity. People understand themselves to have been 'thrown-into' the world. Whereas I believe that we did ask to be born - each of us, individually; we were offered the possibility, and each of us actively chose it. Those who did not so choose, remained as pre-mortal spirits.
Because the baseline assumption of mainstream Christianity is that we were incarnated without consent, this creates a big problem when someone has a life of extreme suffering - because it seems that God has forced that soul to endure extreme suffering, like-it-or-not...
This problem is compounded by the common mainstream Christian assumption that damnation and eternal suffering in Hell is the default outcome of mortal life; and that it is very difficult to avoid this outcome, so that (probably) most people/ nearly everybody ends-up in Hell. Such a nightmare scenario is, to say the least of it, hard to reconcile with God as our loving Heavenly Father... It seems more like an act of gratuitous cruelty, only somewhat ameliorated by the slim chance of eternal happiness.
In other words, under the standard scenario, it is statistically better never to have been born. Yet we are born. ...Which apparently refutes the Christian description of the loving nature of God.
However, if we acknowledge and believe (and know) that we were pre-mortal spirits who freely chose incarnate mortal life en route to resurrected eternal life; then this life can quite simply be made sense of in terms of its being the necessary step of obtaining a mortal body, which can via death become an eternal body; and as a finite period of experience and learning through-which 'theosis' we may (if we choose well) become more-divine in our natures, leading-into in our eternal post-mortal life.
This scenario has a further indirect implication, which is that we would not have been likely to choose our own incarnated mortality unless there was a reasonable chance of avoiding permanent damnation into Hell. It therefore fits with the idea (which I believe to be scriptural) that salvation is a gift (from Jesus) which we must actively reject in order to be self-consigned to a chosen Hell of eternal suffering.
(This is part of the Mormon model of post-mortal life being, for nearly everybody, a matter of levels of 'Heaven'; of essentially pleasing places, with grades from a materialist paradise at the lowest level; to a highest level including close friendship with our brother Jesus, partnership with God our Father, permanent celestial marriage, and participation in the primary work of divine creation... Hell being a reality of eternal suffering chosen by the deliberately unrepentant (prideful) and calculating (Good-hating) sinner: the conscious rejecter of Heaven.)
In sum (and I could say more) it seems to me that the doctrine of pre-mortal spirit life is something close to an essential for a coherent Christian faith if it is to provide a scenario of mortal life that is consistent with our knowledge of God as our loving Heavenly Father who made this world and this life for the benefit of all his children.
I would therefore urge all Christians who do not already have a conviction of pre-mortal life, to consider this matter with seriousness, and diligently to seek personal revelation on the topic.