Karl Barth on monoteism

Do you understand what monotheism in Christian faith means? God knows, not the foolish delight in the number ‘one’. It has nothing to do with the number ‘one’, but with this subject in His sheer uniqueness and otherness over against all others, different from all the ridiculous deities whom man invents. Once we have realised this, we can only laugh, and there is a laugh running through the Bible at these figures. Once the true God has been seen, the gods collapse into dust, and He remains the only One. ‘I am the Lord thy God … thou shalt have no other gods before Me.’ This ‘thou shalt not’ has the force of ‘thou canst not’. He who calls himself god alongside Him becomes the mere shadow of man’s extravagant longing, which has its ill results. And the Second Commandment also becomes quite clear then: ‘Thou shalt not make unto thee any image nor any sort of likeness. Thou shalt not bow down to them nor worship them.’  That too is not a sign of Israelite ways of thinking and there is no philosophical concept of invisibility in the background. But God has Himself done everything in order to present Himself. How should man make an image of Him after He has presented His likeness Himself? A well-intentioned business, this entire ‘spectacle’ of Christian art, well-intentioned but impotent, since God Himself has made His own image. Once a man has understood ‘God in the highest’, it becomes impossible for him to want any imagery in thought, or any other kind of imagery.

Karl Barth, Dogmatics in Outline, 31-32
The Christian understanding of monotheism is a claim, it is not an abstract insistence upon a number regarding something that otherwise is irrelevant or unimportant.
It is the insistence that this ‘God in the highest’ has been and continues to operate uniquely in human history and is intimately interested in its details, claiming, by virtue of existence, a position of immanent significance in relation to anything else that would otherwise occupy our interest.
As such, Christian monotheism is only rightly understood as underlining a pressing reality – it is not about a “number”.