Mar 3:25 And if a house be divided against itself, that house cannot stand.
The only difference in this verse from the previous is the use of the term "house." Lest we forget, but Christ and his earthly father Joseph were house-builders so the idea of a "house" had a rich symbolism for him. (The Greek term describing Christ and translated as "carpenter" is actually closer in meaning to "mason" and refers specifically to someone who builds houses as opposed to working with wood, for example, building furniture.)
As a housebuilder, Christ sees a house as two things.
First, he sees it as a structure whose architecture matters. His first major parable using the idea of a house deals with building it on a solid foundation (Mat 7:24). For him, a house that cannot stand is in a very physical sense, no house at all.
Second, he sees a "house" as a family or a clan, encompassing all the people in the house and headed by the master of the house. For Christ, the world of men is divided into kingdoms and kingdoms are divided into houses (Mat 24:43). Belonging to a given "house" is a matter of personal allegiance (Mat 19:29) to the master. The house can be judged as a group (Mat 10:13). The role of the master of the house is to protect the people and property within the house. (Mat 12:29, Mat 24:43).
So, from Christ's viewpoint, a kingdom has one leader and must be united behind him. Beneath the kingdom, a nation is divided into "houses" where allegiance goes to the master of the house. However, the king has authority over all the houses below him.
On one level, this verse is a simple logical statement about what makes up a house. A member of a house cannot be against the master of the house because his membership in a house depends on his allegiance to the master. Nor can a master of a house be against his (or her, widow's also have houses Mat 23:14) house's members because his authority comes from the responsibility of protecting those members from outsiders. The violation in either half of this contact, the leader's or a follower's, destroys the house itself as a unit.
However, the larger topic here is about adversity and who has authority over it. This is the third level on which Christ has answered this question. His first statement was on the conceptual level, that adversity cannot destroy adversity. His next was about whether God's kingdom was divided between God and His adversaries. Christ also answer this in the negative. Such a kingdom would not be a kingdom at all. Finally, we come down to the third level, where "adversity" could be seen as a "house" within God's kingdom. Again, Christ denies that the leader of that house (Beelzebub) could act against the other members of that house (the demons who are caste out) because that would destroy the very nature of the contract that creates the house.
Who can command a house and its master? Only one person, the king. Christ makes this clear in another verse when he say that the choice between Christ and your "house" is a higher level choice: choosing your king over the master of your house (Mat 19:29). Christ has authority over adversity because it represents a "house" within his kingdom. It is NOT a division within his kingdom not does he represent the house of adversity for the master of adversity.
So, does this mean that adversity can be personified in Beelzebub and spiritual demons who inhabit the house of adversity in the spirit world? Christ doesn't say that and his first statement seems to deny it. He is merely responding to the problem of suffering into the terms that it was presented to him. In this case, what he denies is more important than what he affirms. In other verse, he makes it clear that the type of demons he is talking about can only live within people separately on the spiritual plain.
He first denies that the concept of adversity can be divided into individuals that can be against each other. This seems to mean that adversity should not be personified, but it doesn't go that far.
Next, he denies that adversity represented a division in God's kingdom. He denies that "the adversary" is somehow are war with God.
Finally, he denies that house of adversity is divided against itself: that his authority over adversity comes from the master of the house of adversity. Instead, Christ's authority comes from the kingdom to which the house of adversity belongs.
This tells us that adversity (or what is known as the problem of suffering or evil) is part of God's plan and, as such, Christ has authority over it. What does this mean? That aversity has a role in creation and a job to do. Like the Sabbath, which was made for man, adversity is also made for mankind, though we might not recognize the role it plays. Christ actually explains that job very well elsewhere in the Gospels, but his clear explanation is easily misinterpreted. A hint: he symbolizes the role of adversity in our lives as the same as the role of fire in baking bread.