People sometimes use the term “unfathomable” when it comes to God. There are several definitions of this word. Here’s one:
Impossible to comprehend.
But that doesn’t really make sense since some of those same people talk about experiences that they interpret as God’s presence. So, they’re comprehending something.
Incapable of being fully explored or understood.Oxford Dictionary
That makes more sense. In any event, I like the term “mysterious” better.