In today's gospel, some Sadducees, who say there is no resurrection, came to Jesus and asked Him: for a woman who had married seven times, at the resurrection when they arise whose wife will she be?
Sadducees are different from most other Jews. They only believed in things that were explicitly stated in the Law of Moses. Jesus used the Book of Exodus, a book that they accepted, to refute their arguments.
Jesus questioned if they knew the Scriptures or the power of God. In their minds, death is the end, the dead will not be resurrected by God. Jesus quoted what God said to Moses: "I am the God of Abraham, the God of Isaac, and the God of Jacob". His purpose is to point out that God identifies Himself as God of Abraham, Isaac and Jacob, all of whom have been dead (for hundreds of) years. If they are not alive to God, He would not call Himself their God, for He is the God of living. To this world, they may seem dead, but Jesus tells them that they are living.
Jesus asked the Sadducees how well do they know the Scriptures, His question is also for us. Let us read the Bible frequently and not to let him down.