Luke tells us that the Sadducees did not believe in the resurrection of the dead. This was just not an abstract theological debate amongst scholars who disagreed; the debate over the resurrection of the dead was just as politically charged as the argument over paying taxes to Caesar. The Sadducees rejected resurrection, among other reasons, because resurrection subversively suggested that God was going to bring renewal, that God was eventually going to topple kingdoms, including Rome. Those who have a stake in the status quo, who benefit from things staying the same, do not need God coming to rain on their parade.
Quoting from the Book of Exodus, which is found in the Pentateuch (the first five books of the Old Testament), which were the only books the Sadducees accepted as authoritative, Jesus affirms his belief in this politically subversive doctrine. Of course, most of Jesus' fellow Jews believed in resurrection as well, so while there can be no charge for which to accuse Jesus-- at least, they know where he stands.
The Sadducees present a far-fetched scenario to Jesus:
Teacher, Moses wrote for us that if a man's brother dies, leaving a wife but no children, the man shall marry the widow and raise up children for his brother. Now there were seven brothers; the first married, and died childless; then the second and the third married her, and so in the same way all seven died childless. Finally the woman also died. In the resurrection, therefore, whose wife will the woman be? For the seven had married her."
The Sadducees are referring to the practice of levirate marriage referred to in Deuteronomy 25:5-10:
When brothers reside together, and one of them dies and has no son, the wife of the deceased shall not be married outside the family to a stranger. Her husband's brother shall go in to her, taking her in marriage, and performing the duty of a husband's brother to her, and the firstborn whom she bears shall succeed to the name of the deceased brother, so that his name may not be blotted out of Israel. But if the man has no desire to marry his brother[s widow, then his brother[s widow shall go up to the elders at the gate and say, "My husband's brother refuses to perpetuate his brother's name in Israel; he will not perform the duty of a husband's brother to me." Then the elders of his town shall summon him and speak to him. If he persists, saying, "I have no desire to marry her," then his brother's wife shall go up to him in the presence of the elders, pull his sandal off his foot, spit in his face, and declare, "This is what is done to the man who does not build up his brother's house." Throughout Israel his family shall be known as “the house of him whose sandal was pulled off."
The purpose of levirate marriage was to provide a male heir for the man who died. The Sadducees are not asking Jesus to comment on the practice, but rather to ask whose wife this woman will be in the resurrection. Perhaps they are trying to point out from their perspective the non-sensical nature of resurrection. They have assumed that the resurrected life will basically consist of the current situation with no end.
Jesus dispels their faulty understanding of eternity. It will most certainly be the case that our earthly relationships will still have meaning in the life to come, but we will no longer order our relationships as we do in this life. Since we will finally be one family united in God's completed redemption, we won't make such familial distinctions. We will all be children of God equally and nothing less.
But Jesus does not leave the matter there. He knows that the Sadducees are not really looking for an answer to the question of whose wife this woman will be in the world to come. They are interested in tripping up Jesus to undermine his influence with the crowd. Since most Jews believed in resurrection, Jesus needs to affirm it. So Jesus moves past their question to get to the heart of the matter with some insightful biblical interpretation.
And the fact that the dead are raised Moses himself showed, in the story about the bush, where he speaks of the Lord as the God of Abraham, the God of Isaac, and the God of Jacob. Now he is God not of the dead, but of the living; for to him all of them are alive (see Exodus 3:6).
As the Sadducees quote Moses because he is authoritative in an attempt to show the silliness of the doctrine of resurrection, so now Jesus too quotes Moses in the story of the burning bush to affirm it. When God appeared to Moses on the mountain, he did not speak of the patriarchs in the past. God did not say, "I was the God of Abraham, Isaac, and Jacob. No, God himself affirmed that even though the patriarchs were dead he is still their God. Alyce McKenzie writes,
The Sadducean objection to the resurrection has been shown to be invalid. It is based on the assumption that earthly conditions persist in the heavenly world.
Jesus' countering of their objection is not an affirmation of immortality, but of the fact that God will raise the dead because God cannot fail to keep his promises to them that he will be their God. To say that God will be their God means that they draw their life from him. God is the God of the living not the dead. God is the God of two by two and one by one. God is the God of the old and the young.
Before long, the Sadducees will receive the living proof of Jesus' defense of the Resurrection when he himself dies, but is alive again.
God IS our God for all of eternity. Long after we die, God will still affirm that he IS our God. We are the sons and daughters of God because our brother Jesus is the resurrection and the life.