On page 105 of The Doctrine of God, Herman Bavinck writes regarding Exodus 3:14:
"God does not call himself 'the One who is' in the abstract. He gives no explanation of his aseity, but he declares very explicitly what he is and what is his character. Now what is he and what is his character? This cannot be expressed in a single word, but 'he will be that he will be.' Everything is included in this expression; to be sure this qualification is general and indefinite, but for that very reason it is so rich and so full of meaning: he will be what he has been for the patriarchs, what he is now, and what he will remain: for his people he will be everything."