Jerome

But concerning the resurrection of the dead, have you not read what was spoken by God, saying to you: I am the God of Abraham, the God of Isaac, and the God of Jacob: God is not the God of the dead, but of the living. And when the crowds heard this, they were amazed at his teaching. To prove the truth of the resurrection, he could have used many other more obvious examples, among which is: The dead will be raised up, and those who are in the graves will rise again (Isaiah 26:19). …

Commentary on Matthew ↗

Hilary of Poitiers

It should be further considered, that this was said to Moses at a time when those holy Patriarchs had gone to their rest. They therefore of whom He was the God were in being; for they could have had nothing, if they had not been in being; for in the nature of things that, of which somewhat else is, must have itself a being; so they who have a God must themselves be alive, since God is eternal, and it is not possible that that which is dead should have that which is eternal. …

Catena Aurea by Aquinas ↗

Thomas Aquinas

And how does it serve the purpose? He says, I am the God of Abraham, the God of Isaac, and the God of Jacob. God is said to be the God of certain ones in their worshiping him. These, therefore, worship him. But to worship God does not belong to the dead, but to the living. Therefore Abraham, Isaac, and Jacob are alive; but not according to the body: therefore according to the soul. But what does this avail for the resurrection? …

Commentary on Matthew ↗