If God existed before everything else, then who or what created God? Since nothing else existed, the definitive answer is that nothing could have created God, and its existence is not only ineffable, but logically unsound. Something cannot come from nothing. The universe itself also suffers from this problem. People who posit the existence of God and/or the universe are implying that the very principles of logic are so broken that literally anything follows from an argument. I could say that because some flower pots are red, Australia is made of bricks and it would be just as logical as saying that God or a universe arose from nothing.
Therein lies the problem of saying God “exists”. To exist is to be a part of existence — to be part of the creation. God isn’t a part of existence; God is the ground of existence, or existence itself. Ergo, God doesn’t “exist”.
But that’s only what I’ve gleaned from a cursory reading of Tillich.