Albert Barnes Commentary Matthew 22:31-32

"But as touching the resurrection of the dead, have ye not read that which was spoken unto you by God, saying, I am the God of Abraham, and the God of Isaac, and the God of Jacob? God is not [the God] of the dead, but of the living." — Matthew 22:31-32 (ASV)

Regarding, etc. That is, in proof that the dead are raised. The passage Jesus quotes is recorded in Exodus 3:6 and Exodus 3:15. This event occurred at the burning bush, and Jesus's reference to it is recorded in the Gospels of Mark and Luke. Abraham, Isaac, and Jacob had been long dead when Moses spoke this: Abraham for 329 years, Isaac for 224, and Jacob for 198.

Yet God spoke at that time as still being their God. They must, therefore, still be living; for God is not the God of the dead—that is, it is absurd to say that God rules over those who are extinct or annihilated—but He is the God only of those who have an existence.

Luke adds, All live unto him. That is, all the righteous dead—all those of whom He can properly be called their God—live to His glory. This passage does not prove directly that the dead body would be raised, but only by consequence. It proves that Abraham, Isaac, and Jacob had an existence then, or that their souls were alive. This the Sadducees denied (Acts 23:8), and this was the main point in dispute. If this was admitted—if there was a state of rewards and punishments—then it would easily follow that the bodies of the dead would be raised.