Apparently you do NOT understand it, at all. Jesus was talking to the Pharisees, you know those people He said were of their father, the devil?
Pharisees practiced exorcism, thinking they were casting out demons for God. But the Pharisees were of their father, the devil. Jesus named their practice to disprove their accusation about Him. If you and your disciples cast out demons, then it proves that you are also of the devil. (v. 27-28).
So sorry you missed His whole point. People who are not saved thinking they are doing God's work, when they don't even know God, are casting out demons by their father, the devil. And thinking because they do good works for righteousness, their exorcisms MUST be God's work.
They fell for it then. They are falling for it today. Satan can cast out all the demons he needs to keep someone blind and lost. Know any religion who believe their works are their righteousness, who practice exorcism, and think God is behind their "success"?
Maybe your problem is pigheadedness and not ignorance after all. The Pharisees questioned Jesus about His having cast out demons and suggested that He was acting under Beelzebub's, not God's authority (just like you did). Jesus provided the logical argument that that was ridiculous (just like I did)