The Sadducees did not believe in angels or in the resurrection after death. So, they tried to stump Jesus on the issue of the resurrection (Mark 12:18-23). Jesus pointed out that they didn't know the Scriptures nor the power of God (Mark 12:24). If they knew the Scriptures and the power of God, they would know that there is a resurrection in store for the dead, and they would not have doubts about the power of God to make it happen. The God who created the heavens and earth from nothing, who created man from the dust of the earth, will not be handed a problem with raising all the dead. It will be as easy to Him as forming the universe, the animals and vegetation, and covering the earth instantly with life and vegetation.
There were flawed assumptions that the Sadducees used to justify their religious beliefs, but Jesus did not give compliment to their right to believe as they wanted. There was no pretending that they had their truth and He had His truth, and everybody is fine. They needed to break free of their flawed beliefs. Jesus affirmed that the scriptures did teach a resurrection in the future, and He also affirmed the reality of angels: "For when they rise from the dead, they neither marry nor are given in marriage, but are like the angels in heaven" (Mark 12:25). He told them they were "greatly mistaken" (Mark 12:27). People have a legal right among men to believe what they want, but no one has a divine right to teach what is not true to God's word. Being greatly mistaken about God's power and His scripture teaching can have eternal consequences. Don't be "greatly mistaken" in your beliefs. Get your beliefs only from the scriptures handled correctly and carefully!