Jesus was driving out a demon that was mute, and when the demon had gone out, the mute man spoke and the crowds were amazed. Some of them said, “By the power of Beelzebul, the prince of demons, he drives out demons.” Others, to test him, asked him for a sign from heaven. But he knew their thoughts and said to them, “Every kingdom divided against itself will be laid waste and house will fall against house. And if Satan is divided against himself, how will his kingdom stand? For you say that it is by Beelzebul that I drive out demons. If I, then, drive out demons by Beelzebul, by whom do your own people drive them out? Therefore they will be your judges. But if it is by the finger of God that I drive out demons, then the Kingdom of God has come upon you. When a strong man fully armed guards his palace, his possessions are safe. But when one stronger than he attacks and overcomes him, he takes away the armor on which he relied and distributes the spoils. Whoever is not with me is against me, and whoever does not gather with me scatters.”
Now when the miracle was performed, the multitude extolled Him with loud praises, and the glory which was due to God. As it follows, "And the people wondered." The suspicion of the Pharisees being utterly without reason, they dared not divulge it for fear of the multitude, but pondered it in their minds. Hence it is said, "But he, knowing their thoughts, said to them, 'Every kingdom divided against itself will be brought to desolation.'"
Jesus did not answer them from the Scriptures, since the pharisees gave no heed to them, twisting their meaning; but He answers them from things of every day occurrence. For a house or a city if divided is quickly scattered to nothing; and likewise a kingdom, than which nothing is stronger in that time. For the harmony of the inhabitants maintains houses and kingdoms. If then, says He, I cast out devils by means of a devil, there is dissension among them, and their power perishes. Hence He adds, "But if Satan be divided against himself, how shall he stand?" For Satan does not resist himself, nor hurt his soldiers, but rather strengthens his kingdom. It is then by Divine power alone that Jesus crushes Satan under His feet.
The kingdom of the Father, Son, and Holy Spirit, is also undivided, because it is sealed with an eternal stability. Let those who say that the Son is inferior to the Father, but also that the Holy Spirit inferior to the Son cease, since whose kingdom is one, their power is one also.
Catena Aurea (St. Thomas Aquinas)
Homily 41 on Matthew (St. John Chrysostom)
Commentary on Luke, Sermon LXXX (St. Cyril of Alexandria)
St. Bede the Venerable