From: Luke 20:27-40
The Resurrection of the Dead
 There came to Him (Jesus) some Sadducees, those who say that there is no resurrection,  and they asked Him a question saying, "Teacher, Moses wrote for us that if a man's brother dies, having a wife but no children, the man must take the wife and raise up children for his brother.  Now there were seven brothers; the first took a wife, and died without children;  and the second  and the third took her, and likewise all seven left no children and died.  Afterward the woman also died.  In the resurrection, therefore, whose wife will the woman be? For the seven had her as wife."
 And Jesus said to them, "The sons of this age marry and are given in marriage;  but those who are accounted worthy to attain to that age and to the resurrection from the dead neither marry nor are given in marriage,  for they cannot die any more, because they are equal to angels and are sons of God, being sons of the resurrection.  But that the dead are raised, even Moses showed, in the passage about the bush, where he calls the Lord the God of Abraham and the God of Isaac and the God of Jacob.  Now He is not God of the dead, but of the living; for all live to Him."  And some of scribes answered, "Teacher, You have spoken well."  For they no longer dared to ask Him any question.
27-40. The Sadducees did not believe in the resurrection of the body or the immortality of the soul. They came along to ask Jesus a question which is apparently unanswerable. According to the Levirate law (cf. Deuteronomy 25:5ff), if a man died without issue, his brother was duty bound to marry his widow to provide his brother with descendants. The consequences of this law would seem to give rise to a ridiculous situation at the resurrection of the dead.
Our Lord replies by reaffirming that there will be a resurrection; and by explaining the properties of those who have risen again, the Sadducees' argument simply evaporates. In this world people marry in order to continue the species: that is the primary aim of marriage. After the resurrection there will be no more marriage because people will not die anymore.
Quoting Sacred Scripture (Exodus 3:2, 6) our Lord shows the grave mistake the Sadducees make, and He argues: God is not the God of the dead but of the living, that is to say, there exists a permanent relationship between God and Abraham, Isaac and Jacob, who have been dead for years. Therefore, although these just men have died as far as their bodies are concerned, they are alive, truly alive, in God--their souls are immortal--and they are awaiting the resurrection of their bodies.
See also the notes on Matthew 22:23-33 and Mark 12:18-27.
[The note on Matthew 22:23-33 states:
23-33. The Sadducees argue against belief in the resurrection of the dead on the basis of the Levirate law, a Jewish law which laid down that when a married man died without issue, one of his brothers, according to a fixed order, should marry his widow and the first son of that union be given the dead man's name. By outlining an extreme cases the Sadducees make the law and belief in resurrection look ridiculous. In His reply, Jesus shows up the frivolity of their objections and asserts the truth of the resurrection of the dead.]
[The note on Mark 12:18-27 states:
18-27. Before answering the difficulty proposed by the Sadducees, Jesus wants to identify the source of the problem--man's tendency to confine the greatness of God inside a human framework through excessivereliance on reason, not giving due weight to divine Revelation and the power of God. A person can have difficulty with the truths of faith; this is not surprising, for these truths are above human reason. But it is ridiculous to try to find contradictions in the revealed word of God; this only leads away from any solution of difficulty and may make it impossible to find one's way back to God. We need to approach Sacred Scripture, and, in general, the things of God, with the humility which faith demands. In the passage about the burning bush, which Jesus quotes to the Sadducees, God says this to Moses: "Put off your shoes from your feet, for the place on which you stand is holy ground" (Exodus 3:5).]
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.