In Humanism: A Very Short Introduction, Stephen Law writes:
The view that we cannot say what God is, only what God is not… has its attractions, perhaps the most obvious being that, if you never say what God is, you can never be contradicted.
Mark Vernon extends the thought:
Imagine you have lived all your life in a landlocked country, where there is no talk or sight, let alone comprehension, of the sea. There's not even the word. Then, one day, you venture across the horizon and after a long journey reach the end of land. And you see it. The sea. Astonished, you contemplate the view for a while and then you head back to your fellows. You try to describe what you've seen. It's not land, you begin. It's not hilly or mountainous, you continue. It's not possible to walk across it. It's not covered with grass and trees. It's a bit like that lake, only it has no apparent bounds and it does weird things like approaching the land and then retreating from it, day by day.
You take the point. … Similarly, as Aquinas says of God, it's not that mere mortals can have no knowledge of God, but because by definition what God truly would be lies beyond comprehension, that knowledge will always be provisional and hedged with mystery.
The Dish 