What does Mark 12:26 mean?

The Sadducees are debating with Jesus over whether there is life after death. Jesus is frustrated; while these men purport to be religious experts, they don't understand the Scriptures or God (Mark 12:24). Unlike the Pharisees and scribes, who add manmade traditions to God's Word, Sadducees stick closely to the literal text. Knowing this, Jesus answers from Moses' writings (Exodus 3:6). Moses came upon the voice of God coming from a bush in the wilderness that was engulfed in flames but didn't burn up. The voice identified Itself as the God of the Jewish patriarchs—patriarchs who happened to have been dead for over four hundred years.

Jesus' argument is not based on the present tense verb, because the verb is only implied in God's name, not specifically used. Otherwise the statement would have been "I AM is the God…" His response is based on the nature of God as the living God (Psalm 42:2) and God's choice to be identified by three servants who were no longer living on earth at the time He spoke from the bush. If someone faithfully followed football, watched the teams change every year, kept track of the games, and this person also identified themselves as a fan of a specific team, you could reasonably expect that the team in question still existed.

The implication made is that when God claimed to be the God of Abraham, Isaac, and Jacob, God was implying that such men still existed after their earthly lives had ended.

The Sadducees believe in a more literal translation of the Mosaic law and don't obsess with the oral law, like scribes and Pharisees. Still, the fact that they teach theology at all means they have volunteered to be judged. Later, James will say, "Not many of you should become teachers, my brothers, for you know that we who teach will be judged with greater strictness" (James 3:1). Anyone who teaches the Scriptures needs to know the Scriptures.