First, the Pharisees accompanied by some Herodians. Now some Sadducees, come to Jesus to ask their tricksy question. The import of their question is to ridicule the idea of resurrection.
By the time of Jesus, most Jews believed in the resurrection of the body some time after death. The only place in Hebrew scripture where this is explicitly taught is in Daniel 12. But there are plenty of other writings that were later rejected as canonical that show the common belief in resurrection.
Most people who were not Jewish probably believed in life after death, but more as a bodiless existence in heaven. This was the teaching of many Greek philosophers, and it seems like this was the prevalent thought of the age. Somehow that teaching has never really gone away. Many Christians today envision a bodiless existence in heaven after death. That is not what the New Testament teaches.
Now for the Sadducees. They didn’t believe in resurrection or life after death at all. When you’re dead, you’re dead. They did not accept Daniel as authoritative. In fact, they didn’t really use anything other than the five Books of Moses as authoritative.
A Christian who denies resurrection would rightly be seen as too liberal. Bodily resurrection is an absolute keystone teaching in the New Testament. But Sadducees were not liberal. They were actually too conservative. They could not accept that God continued to reveal himself and thus, more truth, through the continued writings after the first five books of the Bible. It is possible to be too conservative.
Again, I don’t know how much you read the Old Testament, but if you do, you know of the concept of Sheol. The Hebrews had the idea that the most you could look forward to in death was some kind of shadow existence in the place of the dead. The idea of resurrection did not enter until over a thousand years after the time of Moses.
The Sadducees wanted to use the idea of levirate marriage to ridicule the whole idea of resurrection. Levirate marriage, as taught in the law, was all about making sure every man had an heir, so if he died without one his brother would take over and give him one. This particular story, which may be presented as a reality, was almost ridiculous in that the woman had seven brothers and was unable to give an heir.
So, whose wife would she be if there were a resurrection? Levirate marriage was so foundational to the Sadducees that even after death it must certainly pertain.
Jesus first answers the ridicule. The imagination of the Sadducees is too small and their God is too small. They could not envision a life after death that was much more than a continuation of life before death. Jesus says it is way better than that and that our resurrection bodies will be something like the angelic ones. Since there will be no more death, there will be no need for procreation and levirate marriage will no longer pertain. Stretch a little and imagine something bigger and better than what you already know.
Then Jesus answers the question itself. Quoting from Exodus 3, right in the wheelhouse of the Sadducees, Jesus says that God is not God of the dead but of the living.
When Moses received these words at the burning bush, the covenant with the patriarchs was all but forgotten. But not by God. He was renewing the covenant with his people and using Moses to lead them. Abraham, Isaac, and Jacob were not dead and forgotten.
Christians believe in the resurrection. We believe that Jesus was raised from the dead in actuality, and that he lives with a body until this day. We believe that we will be raised at the coming of the Lord and will live with him in the new earth. We don’t know very much about what to expect from the time of death until the time of resurrection, but “with the Lord” is certainly part of what we do expect. What is extremely important is that we expect life after life after death, in the word of NT Wright.