This is the fundamental idea behind the Via Negativa, otherwise know at the Apophatic Way. This is the concpet that because God is so beyond our experience and understanding we cannot hope to talk about him in any meaningful way - and so we should not try. We can only say what God is not: God is not a body, God is not finite, God is immortal etc.
One way of trying to get your head around how different God is to us is laid out by the contemporary philosopher Peter Vardy. He uses the thought experiment of a philosophical fish - this fish is considering what created the fish pond in which he lives. The fish cannot logically conclude that the pond was made by a “super-fish” who is like him in any way - the creator of the pond must be utterly different to the pond. How else would he be able to dig it and fill it with the contents needed? So it is, Vardy argues with the cosmos - all we can say for certain is that God is NOT like us - or indeed like anything within the cosmos.
The Apohatic way is to accept this total difference between us and God, embrace it and wonder at the nature of God as a result. As Aquinas put it in his work Mystical Theology:
The higher we soar in contemplation, the more limited become our expressions of that which is purely intelligible; even as now, when plunging into the Darkness which is above the intellect, we pass not merely into brevity of speech, but even into absolute silence, of thought as well as of words … and, according to the degree of transcendence, so our speech is restrained until, the entire ascent is accomplished, we become wholly voiceless, inasmuch as we are absorbed in Him who is totally ineffable.
Aquinas felt this sense of ineffability keenly - so keenly in fact that a few months before his death in 1274 he stopped writing, refused to continue and declared that all his previous attempts at writing about God were “but straw”. This does not necessarily mean "rubbish" but rather, according the philosopher Mark Vernon, rough and basic rather than refined and accurate. So it was that he had entered into the Via Negativa.
Of course we are still left with a huge amount of his Cataphatic Theology to consider. But to find out what this means you are going to have to wait for a future word of the week!