It is interesting to speculate how the incoherent and mass-apostasy-inducing Omni-God concept arose and came to dominate Christian (and some other) theologies and Priesthoods.
One God who is omnipotent, omniscient, omnipresent and created everything from nothing - i.e. the officially mandatory nature of God according to mainstream Christianity; is an idea literally inconceivable to simple people and children.
And inconceivable people of the past, and up to the life of Jesus; because I see nothing explicit to suggest that the authors of Old Testament, the Gospels, or Epistles conceptualized God in this way.
The Omni idea seems to have emerged - off the record - in the generations after the ascension of Jesus and deaths of the Apostles; until we see it behind the Christology schisms and disputes, concerning the divine and/or human nature of Christ; that tore-apart the nascent church in its early centuries.
Such disputes were (and are) inevitable when Jesus was assumed to have been incarnated into the creation of an Omni-God, since the Omni definition of God excludes the possibility of Jesus being a second creator-deity; and indeed removes any possible reason or purpose for a further manifestation of deity.
(If God is supposed to be able to do everything, know everything, be everywhere etc - then there is no necessity for Jesus Christ to be incarnated, live, die, be resurrected, ascend to Heaven etc. The Omni-God can - by definition - do everything that is doable, so Jesus is superfluous.)
Yet if Jesus Christ is not a truly divine creator God; then Jesus is "just" a teacher, prophet etc - and we either assume Jesus was a false teacher and revert to Judaism; or assume Jesus was a valid teacher as with Islam.
But if Jesus is not divine, he is not unique; and there is no reason to build a religion around following him.
So how and why did such a strange and destructive idea emerge?
My best guess of how, is that the Omni-God was synthesized from the ancient Hebrew idea of a single dominating personal God to whom all other Gods were inferior and subservient; and to whom all worship and obedience was due.
This "monotheism" was combined with Greek philosophical ideas - and religious ideas - probably from elite and initiatory mystery religions; such as Neo-Platonists and Gnostics); of a universal but impersonal abstract-deity that was everything (pantheism), was the ultimate cause of everything, was perfect hence time-less etc.
(Much like some Hindu and Buddhist philosophical concepts of deity.)
But why did the Omni idea happen at all?
My understanding is that this was part of the development of human consciousness; and the increasing inner sense of separation of the individual from the group.
Before the Omni-God concept; human consciousness was naturally, spontaneously, and largely un-consciously very much immersed in the group consciousness - and this group consciousness included the realms of spirits and the divine.
Because people experienced relationships with so many other Beings (such as animals, trees, landscape, the dead, angels, demons, and gods); there was no subjective "problem" of creation as being incoherent.
There was no need to "explain" how it was that the universe held-together and had purpose - because this coherence was simply a matter of experience. Men knew that things cohered.
This was, and still is, how young children experience reality even nowadays; and perhaps some tribal peoples as well.
In other words, in ancient times there was no felt-problem of incoherence that the Omni concept was needed to solve.
There was no need or reason to explain why all the different aspects of reality held-together and had an overall purpose.
And no reason to explain why there were real values - a real and universal morality... Since such things were obvious.
It was when Man's consciousness began (and continued) to become ever more self-aware and separated from the group consciousness; and from awareness of the dead, spirits of many kinds, and the divine - that Men sought an explicit explanation of the "unity" of all things...
That intellectuals came-up with the Omni-God explanation.
This does not explain why the early Christian church seized upon the Omni-God concept and made it mandatory, despite all the problems it has caused ever since.
But I think that ultimately such reasons are social rather than philosophical: to do with the church as an institution, rather than Christianity as a theoretical construct.
The Omni-God concept is popular in all the major monotheisms, I think because it is interpreted to make the specific religion both universal and mandatory; it is used to insert the religion and its church into the ultimate creation and operation of absolutely every-thing.
Omni-God removes choice from religion; and religion becomes a matter of acknowledging one truth for everything...
Or else the act of rejecting that church and religion gets conceptualized as rejecting... absolutely everything, and all possibility of anything.
So, with Omni-God; not be be a member of the church and religion is therefore conceptualized as a wholly irrational and incoherent act - an embrace of absolute nothingness.
If it is broadly correct that the Omni-God emerged after Jesus and in the early centuries AD; driven from-within by changes in Mens' consciousness; then we can also see that a genuine, living, motivating belief in Omni-God has become impossible in the modern era.
Shoals of people have left the Christian church, and/or had their faith reduced to the feebleness of a lifestyle-choice, by the insistence of Christian theology on the Omni-God combined with the gross incoherence of the concept - which incoherence has now become spontaneously obvious even to those of low intelligence.
(The two obvious problems about a Christianity rooted in the Omni-God concept and a Good God; are an inability to explain genuine evil and extreme and undeserved suffering and death; and also an inability to explain human agency or "freedom".)
In replying to a basic question of "who was Jesus?": mystery-word-spells about Jesus being simultaneously wholly-Man and wholly-God seem like meaningless or devious abstractions; when materialism is built-into the modern perspective.
In replying to what did Jesus do?": kick-the-can pseudo-explanations about Jesus being necessary to save Mankind from the blame attached to an "original sin" leading to a "fall" that occurred in prehistory; make no sense (and seem grossly unjust)...
When Man's consciousness has become so spontaneously separated from the group/ spirit/ divine that the choices and events of putative ancient ancestors is experienced as utterly irrelevant to me-here-now.
If the Omni-God was not original to Christianity but instead the contingent artefact of a particular stage in Man's historical consciousness and the rhetorical requirements of the ancient church...
And since the O-G concept currently stands in the path of any possibility of explaining (clearly and simply) the nature of Jesus and what he did...
Then Christians really need to go back and re-examine and reject this Omni-God dogma...
And then we need to replace the obsolete and harmful Omni-God with a conceptualization of God that makes explanatory-sense to us each, here, and now; and which clearly and simply explains the necessity of Jesus Christ for those who desire salvation.