There is snare in what I might call dogmatic primary monotheism. That is, the assumption of 'strict' philosophical monotheism - with monotheism regarded as a rational necessity and a transparent and absolute requirement of Christianity.
For many Christian theologians, it is assumed we know what monotheism is better than we know what Christianity is; and therefore the definition of Christianity must be fitted to this prior understanding of the definition of monotheism.
This is a big problem for mainstream Christian theology, and has been for nearly 2000 years; because when monotheism is combined with an abstract definition of God as necessarily omnipotent, this creates intractable problems for understanding Christianity as it may be understood from a common-sense reading of the Gospels in particular, but also the Bible as a whole.
For example, if there is strict monotheism and an omnipotent God, there is no necessity for Christ. One God who has absolute power would not need the assistance of Christ.
Yet, Biblical revelation is clear that Christ is necessary for Man's salvation - this (it seems to me) is absolutely fundamental to Christianity: a Christian must believe that Christ is necessary to eternal life. And if Christ is necessary to salvation/ eternal life, then God is either not one, or not omnipotent. Something must yield precedence.
This ought not to be a problem for Christians, since it would seem obvious that philosophical conceptualisations such as a particular understanding of monotheism and the abstract and absolute definition of omnipotence are not (not straightforwardly) supported by scripture, while the necessity of Christ has multiple support from scripture.
But historically it certainly has been a problem; because strict monotheism and strict omnipotence have in practise been regarded as primary and non-negotiable. Mainstream theologians have been more willing to fudge, complexify (with apparent contradiction) and in general mis-explain the necessity of Christ; than to challenge, drop or nuance their philosophical attachment to monotheistic omnipotence.
Yet polytheism of the pagan Hindu, Buddhist or other type is clearly not an answer either. In the first place, scripture is clear that (in some fairly plain sense) there is One God whose power is (in some obvious way) primary.
But also that a straightforwardly polytheistic universe is one without focus, unity and overall meaning or purpose: a polytheistic universe just is. It doesn't mean anything, it isn't going anywhere in particular; there are no real objective relationships between the gods, between gods and men, or between men.
In polytheism 'stuff happens' - and our job (each of us, as individuals) is merely to cope with this as best we may.
But for Christianity God rules the world and there is One God; and it is the one-ness of God that makes possible the objective metaphysical (ie. structuring) cohesion of God, gods and Man - but God needs Christ.
There is One God, but Christ cannot be identical with God.
Thus God is significantly constrained in His primary work, and is not absolutely and abstractly 'omnipotent'.
For Christians (and this may be hard to grasp or acknowledge) the world could have been meaningless and purposeless - so God is not a logical necessity to existence. It is God that makes the universe meaningful and purposeful; and it is God that makes relationships real - and these positive attributes arise from God being a person.
God is a person; and persons are constrained, discrete, bounded, concrete - no matter how large in power, God is not an abstraction, not a force nor a logically-derived construct.
Our God is a real, living and loving person, and He is our Father; this we know, and this is primary - whatever else He may be - which matters are up-for definition, discussion, modification.
As such, it follows that God needs help if He is to achieve some purposes; and indeed, as a person He wants, appreciates, loves help from other persons - primarily and necessarily Christ.
In order to save Man, God needed Christ; who is His son, and we also are His sons and daughters.
Exactly the need for Christ, the mechanism by which Christ saved Man, the similarities and differences between Christ being God's son and me and everyone else being God's children - these matters are not crystal clear; and explaining these constitute some of those matters up-for definition, discussion and modification.
But whatever our explanation of these matters, we will need to set-aside the classical, philosophical and traditional metaphysical assumptions that we know and understand that One God means strict monotheism; and that omnipotence means that God can do anything do-able.
These assumptions cannot be true, if the primary revelations of Christianity really are regarded as primary: since God needed Christ - God is not monotheistically-omnipotent.
Note added: Another, quite different, reason to suggest that God needed Christ is to compare Christianity with "pure" monotheisms - Christianity promises a far happier and higher life beyond death: a life of divinisation. If we assume that religions are (broadly) honest about what they offer (which I believe to be the case) then this suggests that One God without Christ cannot offer as much as God with Christ. Therefore, if there had been no Christ, then Man's future would have been much less than it is.