What does Mark 3:24 mean?

Scribes, Pharisees who are experts in the Mosaic Law, come down from Jerusalem to tell the people that Jesus' authority to cast out demons comes from Satan. Jesus responds to their fallacious reasoning with a parable that His audience would find very applicable. Jesus is saying that if He uses Satan's power to rebel against him, Satan's kingdom can't stand.

This idea—specifically taken from the next verse—became famous in American history when used by President Abraham Lincoln during the U.S. Civil War. There, Lincoln made the same fundamental point as Jesus: any power that fights against itself is bound to lose. Lincoln's intent was to show the importance of a united nation. Jesus' intent, here, is to point out that it would be foolish for Satan to use his own power to interfere with demons.

The idea of a "divided kingdom" is also poignant for the people hearing Jesus' words. Israel itself was once united, and strong. When the Israelites escaped Egypt and came to the Promised Land, they divided the territory into twelve tribes (Joshua 13:8—19:51). Eventually, they begged God for a centralized king (1 Samuel 8:4–5). God gave them Saul (1 Samuel 10:17–27), who became so corrupt God rejected Saul's line and chose David to replace him (1 Samuel 15:10–35; 16:1–13). David's son Solomon reigned next (1 Kings 1:28–53).

Then, after Solomon, political unrest caused the northern tribes to rebel against Solomon's son and split off into a new nation (1 Kings 12:16–20). The borders remained more or less steady until the Northern Kingdom of Israel was conquered by the Assyrians (2 Kings 17:7–18), and then the Southern Kingdom of Judah was taken into exile by Nebuchadnezzar (2 Kings 25).

The Jews returned from Babylon and settled in what had been Judah (2 Chronicles 36:22–23). The territory of Galilee, however, sits right in the middle of what had been the Northern Kingdom of Israel. Four hundred years after the last Old Testament prophet, the kingdom is still divided—with non-Jewish Samaria in between—and Rome's occupation means it is not standing independently.