CCC 206

In revealing his mysterious name, YHWH ("I AM HE WHO IS", "I AM WHO AM" or "I AM WHO I AM"), God says who he is and by what name he is to be called. This divine name is mysterious just as God is mystery. It is at once a name revealed and something like the refusal of a name, and hence it better expresses God as what he is - infinitely above everything that we can understand or say: he is the "hidden God", his name is ineffable, and he is the God who makes himself close to men.