By and large, modern Man can no longer accept an hypothesis of instantaneous creation; and there is little doubt that, consciously or subconsciously, he applies that as much to his own individuality as to anything else.
If belief in his own existence is to involve believing that he was created at, or immediately before, his physical birth, he will abandon that belief - and he will have good reasons for doing so.
Edited from Self and Reality, in The Rediscovery of Meaning and other essays, by Owen Barfield (1977).
Barfield is surely correct in his observation of modern metaphysics - there is an inbuilt assumption that things 'gradually come into being'. Among mainstream Christians who have noticed this, such an assumption is typically regarded as necessarily a secular corruption, and there is an attempt to restore a pre-modern (medieval) metaphysics of instantaneous creation from nothing.
But for both Barfield (an Anthroposophist) and myself (a believer in Mormon theology), the 'modern' evolutionary thinking is correct, and in its essence an aspect of the spiritual progression of Man.
For both Anthropososphists and Mormons; the pre-modern belief is instantaneous creation is seen as an error, inherited from pre-Christian classical philosophy.
By 'evolution' is Not, of course, meant the purposeless evolution of Darwinian natural selection; but evolution in its original (and primary) meaning of a purposive and developmental unfolding towards a goal.
The implication is that each us us has an existence stretching back into pre-mortal life. For Barfield, this pre-existence was one of several incarnations with periods of spirit life in between - in which incarnations began as diffuse and spiritual, and became more solid and embodied. (The future envisaged was one of further progressive incarnations, eventually leading to full deity.)
For Mormons; pre-existence began with some kind of primordial consciousnesses, a stage of being spirit children of God, then a chosen incarnation into this mortal life. (The future envisaged is of a post-mortal resurrected eternal life, with ultimate potential for full and embodied deity.)
A belief in pre-existence - that we each, each of us, lived as spirit children of God, before being incarnated in mortal bodies to experience death, and thereby enable resurrection - is a metaphysical assumption that has potential to make a very large and positive difference to Christian life: it is an assumption that overcomes some of the stumbling-blocks and solves many of the deepest problems and paradoxes experienced by modern Christians.
Much more on this vital theme can be read in When Souls Had Wings: Pre-Mortal Existence in Western Thought by Terryl Givens, 2009.
The Mormon understanding is articulated by a recent Apostle (and great orator!) Neal A Maxwell: https://www.lds.org/ensign/1985/11/premortality-a-glorious-reality?lang=eng&_r=1