John Chrysostom

And again by Moses doth He stop their mouths, since they too had brought forward Moses; and He saith, "But as touching the resurrection of the dead, have ye not read, I am the God of Abraham, and the God of Isaac, and the God of Jacob? He is not the God of the dead, but of the living." Not of them that are not His meaning is, and that are utterly blotted out, and are to rise no more. For He said not, I was, but, I am; of them that are, and them that live. …

Homily on the Gospel of Matthew 70 ↗

Augustine of Hippo

God is therefore called in particular The God of Abraham, the God of Isaac, and the God of Jacob, because in these three are expressed all the modes of begetting the sons of God. For God begets most times of a good preacher a good son, and of a bad preacher a bad son. This is signified in Abraham, who of a free woman had a believing son, and of a bondslave an unbelieving son. …

Catena Aurea by Aquinas ↗

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 ↗

Thomas Aquinas

But as concerning the resurrection of the dead, etc. After he showed that they were ignorant of the power of God, here he shows that they were ignorant of the Scriptures. Hence, have you not read that which was said by God, saying to you: I am the God of Abraham, and the God of Isaac, and the God of Jacob? This is written in Exodus 3:6. But Jerome asks, since there are other authorities more explicit about the resurrection, as is found in Isaiah 6 and Ezekiel 33 and Daniel 12, why did he cite this one which is ambiguous? …

Commentary on Matthew ↗