Mark 12:26-27
26Now about the dead rising—have you not read in the book of Moses, in the account of the bush, how God said to him, ‘I am the God of Abraham, the God of Isaac, and the God of Jacob’[d]? 27He is not the God of the dead, but of the living. You are badly mistaken!”
Jesus is responding to the Sadducees with their roundabout question of whether the resurrection is true or not – they didn’t believe in it and wanted to find out what Jesus believed. I think all the talk about husbands and wives was just a red herring. But Jesus responds that the resurrection of the dead is indeed true; after all, when God announced himself to Moses, he said he was the God of Abraham, Isaac, and Jacob. What’s the significance here? If those guys were dead, then it does God no favors to continue to be their God. They can’t worship him. They have no influence anymore. But because God continues to identify himself with Abraham, Isaac, and Jacob, Jesus is making the argument that those three patriarchs must still be alive after death.