What does Mark 3:26 mean?

Jesus doesn't always explain His parables outside the company of the disciples (Mark 4:34), but in this case He does. Like a divided kingdom or a split family, Satan can't work at cross-purposes to himself and maintain what authority and dominion he has.

This infers that Satan has some quantity of authority and dominion to lose. When Adam and Eve follow Satan's suggestion instead of God's command by eating the fruit from the tree of knowledge of good and evil, they condemn their descendants to suffer under Satan's power (Genesis 3:1–7). Satan is the "ruler of this world" (John 12:31) and the "prince of the power of the air, the spirit that is now at work in the sons of disobedience" (Ephesians 2:2). The world lies in his control (1 John 5:19), and unbelievers are bound to him (2 Timothy 2:26). This is why he has the authority to offer Jesus "all the kingdoms of the world and their glory" (Matthew 4:8–9).

But Satan's dominion is far from absolute. Job 1:6–12 shows that whatever Satan does is under God's sovereign authority. He cannot do anything without God's permission, and God will turn anything he does into good for those who follow God (Romans 8:28).

Nor is Satan's authority eternal. Immediately after Adam and Eve doom their children to be born under Satan's control, God promises that one of Eve's offspring will crush Satan's head (Genesis 3:15). Jesus fulfills this prophecy when He dies for our sins and is raised again. The promise will be entirely fulfilled when God throws Satan into the lake of fire forever (Revelation 20:10).