“He is who He is, the same yesterday, today, and forever. This meaning is further explained in Exodus 3:15: YHWH—the God of your fathers, the God of Abraham, Isaac, and Jacob—sends Moses, and that is His name forever.
God does not simply call Himself “the One who is” and offer no explanation of His aseity, but states expressly what and how He is.
Then how and what will He be? That is not something one can say in a word or describe in an additional phrase, but “He will be what He will be.”
That sums up everything. This addition is still general and indefinite, but for that reason also rich and full of deep meaning.
He will be what He was for the patriarchs, what He is now and will remain: He will be everything to and for His people.
It is not a new and strange God who comes to them by Moses, but the God of the fathers, the Unchangeable One, the Faithful One, the eternally Self-consistent One, who never leaves or forsakes His people but always again seeks out and saves His own.
He is unchangeable in His grace, in His love, in His assistance, who will be what He is because He is always Himself.”
–Herman Bavinck, Reformed Dogmatics: God and Creation, Vol. 2 (Ed. John Bolt, and Trans. John Vriend; Grand Rapids, MI: Baker Academic, 2004), 2: 143.