It is an old question - revived for me by listening to an audiobook CS Lewis essay on the topic, where he makes a rational argument for the necessity of monotheism.
On the face of it, Christianity is not a monotheistic religion because of Jesus Christ; who is God - but not the only God, not the same God as the already-existing God of the Ancient Jews to whom Jesus frequently refers, defers and prays.
But for some reason earlyish (probably the second century of the religion) many of the most intellectually sophisticated Christian theologians began to regard it as absolutely necessary that Christianity should be monotheistic as well as having at least two Gods.
The question is, why did they feel that way? I infer that it was because the philosopher-theologians were also (and already) embarked on a philosophical quest to explain the coherence of reality and the necessity of God; they wanted to unite all reality in a single unity that was also deity. They were engaged in a conflation of Christian theology with philosophy, because they simply took the absolute necessity of their philosophy for granted.
(And they ended by shoe-horning Christian theology into classical philosophy; while also modifying that philosophy somewhat in the process.)
It seems to me that many philosophical Christians simply assume that this is necessary to the religion - i.e. that for Christians God must be ultimately necessary to be the source of everything and all order, and also that for reality to hang together requires that God be not only one, but indivisibly one. Hence monotheism.
I don't accept this line of argument, because - like all lines of argument - it has assumptions; points at which we must assert It Just Is - but these assumptions are not intrinsic to Christianity and instead come from outside it.
And there are other assumptions which work just as well as Christian explanations, are simpler, more comprehensible and have better implications.
So we can drop the necessity for monotheism and suggest that the coherence of reality comes from other sources - especially that there is one creation (not creation by one, but one creation) in which we dwell. This creation (and its creator/s) is, of course, not logically entailed; but a thing which happened-to-have-happened.
We then understand the one-ness of God as described in scripture to be the one-ness of a King, a reference to primacy not unity of identity - and we find that this fits comfortably with the Old Testament culture, language and descriptions.
And Christians are to understand the cohesion of reality to be due to Love - in some sense of Love (and I tried to describe my own understanding in a post yesterday); which is a inter-personal thing, which means that the universe of reality is personal from top (from God) to bottom ('non-living' matter): an alive and conscious universe of manifold entities, cohering by love.
So we end with a very different world picture from classical theology. And this world picture is not monotheistic - but instead explains monotheism as a consequence of philosophical (not Christian) compulsioins.
But, as an explanation non-monotheism works at-least-equally well: indeed I would assert that it works better for Christians.