John Calvin Commentary Isaiah 43:21

"the people which I formed for myself, that they might set forth my praise." — Isaiah 43:21 (ASV)

This people have I created for myself. The Prophet means that the Lord will necessarily do what he previously said, because it concerns his glory to preserve the people whom he has chosen for himself. Therefore, these words are intended for the consolation of the people. “Do you think that I will allow my glory to fall to the ground? It is connected with your salvation, and therefore your salvation will be the object of my care. In a word, know that you will be saved, because you cannot perish unless my glory also perishes. You will therefore survive, because I wish you to continually proclaim my glory.”

When he says that he has created the people, let us learn that it proceeds from supernatural grace that we are the people of God. For we must remember that principle of which we have previously spoken: that he is not now speaking of ordinary human nature, but of spiritual regeneration, or of the adoption by which he separates the Church from the rest of the world, along with everything that belongs to it. Let no one, therefore, ascribe his regeneration to himself or to any human merits; instead, let us acknowledge that we owe such a great favor entirely to the mercy of God.

They shall declare my praise. Although it is the Prophet's design to show what I have said—that his people will be saved because it concerns the glory of God—we also learn from this that the purpose of our election is that we may show forth the glory of God in every possible way (Exodus 14:4, 17, 18).

The reprobate are, indeed, the instruments of the glory of God, but it is said to shine forth in us in a very different manner. For, as Paul says, he hath chosen us that we might be holy and without blame before him through love, who predestinated us that he might adopt us to be his children through Jesus Christ, in himself, according to the good pleasure of his will, to the praise of the glory of his grace by which he hath made us accepted through the Beloved (Ephesians 1:4–6).

Such also is the import of the words of Peter, when he says that we were brought out of darkness into the wonderful kingdom of God, that we may declare his perfections (1 Peter 2:9); and likewise the words of Zacharias: That, being delivered out of the hand of our enemies, we may serve him without fear, in holiness and righteousness, all the days of our life (Luke 1:74–75).

This, then, is the purpose of our calling: that, being consecrated to God, we may praise and honor him during our whole life.