… What is meant by the ‘one’ in mono-theism? Does it mean ‘one’ as opposed to two, three, or seventy-nine? In which case it is one as a number, and is opposed to other numbers. In that case, since whenever we define something over against something, it is true to say that it is much more like that thing than it is unlike it, ‘one’ God would be merely a uniquely big, powerful and somewhat lonely member of the series of ‘gods’ all of whose other members have been declared inexistant.
But there is another use of the word ‘one’, which is not properly speaking a numerical use at all. This is where ‘one God’ is opposed to ‘nothing’. In other words, where ‘one’ is more like the exclamation ‘is!’ than it is like a number. The exclamation ‘is!’ is opposed to ‘nothing there!’ Now just as the number ‘one’ is more like the other numbers that it is scrubbing out than it is different from them, so the ‘one God’ as opposed to ‘nothing at all’ is more like the ‘nothing at all’ that it is opposed to than it is to anything else. In other words, following this understanding of the ‘mono’ in ‘monotheism’, God is much more like ‘nothing at all’ than like ‘one of the gods’.
And this, of course, is part of the genius of monotheistic Judaism: the realisation that ‘one God’ is much more like ‘no god at all’ than like ‘one of the gods’. In other words that atheism, which is untrue, offers a much less inadequate picture of God than theism, which is true. For monotheistic Judaism, as for monotheistic Catholicism, which I take to be universal Judaism, the principal temptation is not atheism, but idolatry.
James Alison, “Monotheism and the Indispensibility of Irrelevance”, in Underground God: Dispatches from the Scene of a Break-In (New York / London: Continuum, 2006), p. 18.