How can Satan cast out Satan? Observe here that our Lord distinctly affirms the personality of Satan, and a real kingdom of evil. But then he goes on to show that if this their allegation were true, namely, that he cast out devils by the prince or the devils, then it would follow that Satan's kingdom would be divided against itself.
As a house divided against itself cannot stand, so neither could the kingdom of Satan exist in the world if one evil spirit was opposed to another for the purpose of dispossessing, the one the other, from the minds and bodies of men.
Our Lord thus employs another argument to show that he casts out evil spirits, not by Beelzebub, but by the power of God. It is as though he said, "As he who invades the house of a strong man cannot succeed until he first binds the strong man; in like manner I, Christ Jesus, who spoil the kingdom of Satan, whilst I lead sinners who had been under his power to repentance and salvation, must first bind Satan himself, otherwise he would never suffer me to take his captives from him.
Therefore he is my enemy, and not in league with me, not my ally in the casting out of evil spirits, as you falsely represent me to be. It behoves you, then, to understand that it is with the Spirit of God that I cast out devils, and that therefore the kingdom of God is come upon you."