"Then the Sadducees, who say there is no resurrection, came to him with a question. 'Teacher, ' they said, 'Moses wrote for us that if a man's brother dies and leaves a wife but no children, the man must marry the widow and raise up offspring for his brother. Now there were seven brothers. The first one married and died without leaving any children. The second one married the widow, but he also died, leaving no child. It was the same with the third. In fact, none of the seven left any children. Last of all, the woman died too. At the resurrection whose wife will she be, since the seven were married to her?'" NIV translation
The Sadducees were the liberals of the day and believed that this life was all there was. They came trying to trap Jesus and cause a big controversy. It was really odd for them to come and ask about something that they did not even believe happened.
"Jesus replied, 'Are you not in error because you do not know the Scriptures or the power of God?'" NIV translation
Jesus used a nice way of telling the Sadducees that they did not know what they were talking about. He also pointed out that they did not even know the power of God because they did not believe that He could raise men from the dead.
"'When the dead rise, they will neither marry nor be given in marriage; they will be like the angels in heaven. Now about the dead rising - have you not read in the Book of Moses, in the account of the burning bush, how God said to him, "I am the God of Abraham, the God of Isaac, and the God of Jacob"? He is not the God of the dead, but of the living. You are badly mistaken!'" NIV translation
Jesus answered their question by explaining that, in heaven, there are no more people being born. Our physical bodies are no more and we have a new and improved physical body. He also went on to explain that, when God spoke to Moses, He confirmed the fact that there will be a resurrection.
This is a fundamental principle of the gospel as the resurrection of Jesus is the basis for our resurrection to eternal life.