This is true with any kingdom or government or even one's own house. If you can divide a house or kingdom, it becomes vulnerable and weak and can be easily destroyed. Conversely, unity fortifies a house or kingdom. There is strength in unity. Solomon said in Eccl.
Division is destructive.
Besides, Jesus makes it very clear in verse 27 that he was stronger than the devil. Now he is taking back the spoil of human souls that the devil had taken at his will since the beginning. In verses 28-30, Jesus points out the danger of the position they had taken. Vine also explains that what Jesus is saying in Mark 3 is that anyone with the evidence of the Lord's power before their eyes and still declares it to be satanic, has exhibited a condition of heart beyond divine illumination and are therefore hopeless. For he that cometh to him must believe that he is and that he is a rewarder of them that diligently seek him.