The demons are a kingdom divided against themselves. Jesus is addressing a challenge that the demon He had just cast out of this person had been cast out by Satan. Jesus' argument is, "No, Satan would never cast out Satan." It would be stupid for Satan to cast himself out. He is not saying that under every condition Satan or other demons will not cast out other demons. Indeed, that does occasionally take place. They are very capable of doing signs and lying wonders. They can make it look as though somebody has been healed, when God has not done the healing at all, but simply by the removal of one demon by a demon of greater power.
One of the things that saves us is that the demons are divided against themselves. Because they are a kingdom divided against themselves, they cannot stand—they cannot get their act together because their character is such that they are always in competition with each other.
We can understand this when we recognize that the governments and most of humanity has been subject to and deceived by demons. Carnal, human nature, is a reflection of the nature of Satan and his demons. What fruit does that produce among men? Can men get along? No. The other side of the coin is that the beings who inspire, guide, direct, or motivate men not to get along with one another cannot get along with themselves either! The only thing that keeps them unified is that at the head of this organization is a demon of such awesome power that he is able to whip them into line from time to time to carry out his bidding. He does it by sheer force. They do not serve in love of him.
They are a kingdom divided against themselves. They will fall and that is an advantage to us. Being rebellious, they are disorganized. They cannot get their act together. Far more important is they know God exists, and they tremble before Him. They are therefore restrained.