From: Luke 11:15-26:
The Kingdom of God and the Kingdom of Satan
(Now Jesus was casting out a demon that was dumb; when the demon had gone out, the man spoke, and the people marvelled.)  But some of them said, "He casts out demons by Beelzebul, the prince of demons";  while others, to test Him, sought from Him a sign from Heaven.  But He, knowing their thoughts, said to them, "Every kingdom divided against itself is laid waste, and house falls upon house.  And if Satan also is divided against himself, how will his kingdom stand? For you say that I cast out demons by Beelzebul.  And if I cast out demons by Beelzebul, by whom do your sons cast them out? Therefore they shall be your judges.  But if it is by the finger of God that I cast out demons, then the Kingdom of God has come upon you.  When a strong man, fully armed, guards his own palace, his goods are in peace;  but when one stronger than he assails him and overcomes him, he takes away his armor in which he trusted, and divides his spoil.  He who is not with Me is against Me, and He who does not gather with Me scatters."
 "When an unclean spirit has gone out of a man, he passes through waterless places seeking rest; and finding none he says, `I will return to my house from which I came.'  And when he comes he finds it swept and put in order.  Then he goes and brings seven other spirits more evil than himself, and they enter and dwell there; and the last state of that man becomes worse than the first."
14-23. Jesus' enemies remain obstinate despite the evidence of the miracle. Since they cannot deny that He has done something quite extraordinary, they attribute it to the power of the devil, rather than admit that Jesus is the Messiah. Our Lord answers them with a clinching argument: the fact that He expels demons is proof that He has brought the Kingdom of God. The Second Vatican Council reminds us of this truth: The Lord Jesus inaugurated His Church by preaching the Good News, that is, the coming of the Kingdom of God, promised over the ages in the Scriptures [...]. The miracles of Jesus also demonstrate that the Kingdom has already come on earth: "If it is by the finger of God that I cast out demons, then the Kingdom of God has come upon you (Luke 11:20); cf. Matthew 12:28). But principally the Kingdom is revealed in the person of Christ Himself, Son of God and Son of Man, who came `to serve and to give His life as a ransom for many' (Mark 10:45)" (Vatican II, "Lumen Gentium", 5).
The strong man well armed is the devil, who has enslaved man; but Jesus Christ, one stronger than he, has come and conquered him and is despoiling him. St. Paul will say that Christ "disarmed the principalities and powers and made a public example of them, triumphing over them" (Colossians 2:15).
After the victory of Christ the "stronger one", the words of verse 23 are addressed to mankind at large; even if people do not want to recognize it, Jesus Christ has conquered and from now on no one can adopt an attitude of neutrality towards Him: he who is not with Him is against Him.
18. Christ's argument is very clear. One of the worst evils that can overtake the Church is disunity among Christians, disunity among believers. We must make Jesus' prayer our own: "That they may be one; even as Thou, Father, art in Me, and I in Thee, that they may also be one in us, so that the world may believe that Thou hast sent Me" (John 17:21).
24-26. Our Lord shows us that the devil is relentless in his struggle against man; despite man rejecting him with the help of grace, he still lays his traps, still tries to overpower him. Knowing all this, St. Peter advises us to be sober and vigilant, because "your adversary the devil prowls around like a roaring lion, seeking some one to devour. Resist him, firm in your faith" (1 Peter 5:8-9).
Jesus also forewarns us about the danger of being once more defeated by Satan--which would leave us worse off than were before. The Latin proverb puts it very well: "corruptio optimi, pessima" (the corruption of the best is the worst.) And St. Peter, in his inspired text, inveighs against corrupt Christians, whom he compares in a graphic and frightening way to "the dog turning back to his own vomit and the sow being washed and then wallowing in the mire" (cf. 2 Peter 2:22).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.