Jesus certainly implies that Satan casting out Satan would be purposeless and counter to his (Satan's) goals.
Matthew 12:
26 If Satan drives out Satan, he is divided against himself. How then can his kingdom stand?
Understanding the context in which Jesus speaks here, the phrase
"casting out Satan" connotes Satan being forced to cede power and authority. This is entirely counter to Satan's goals, which is to increase his power and authority.
Satan's kingdom will not stand ... because Jesus is casting him out.
In Matthew 7, those that approach Jesus only
CLAIM to have done His works, including the casting out of demons. I am surprised to find that so many who read this passage find it difficult to discern that those with whom Jesus has had no relationship (i.e. "I never knew you") ... are quite apt to be liars ... or to be deceived themselves.
Finally, I would say that he who John reports had been casting out demons ... was casting them out in Jesus' authority, ... although
he was not a part of Jesus' immediate following at that time (i.e. one of the twelve).
I think that the immediate lesson from this scene from Jesus' earthly ministry teaches us that all who have unity with Jesus ... may not, in fact, be part of
our group (think
"denomination").
This idea is further evidenced in the early ministry of Paul. Quite soon after the restoring of his sight after his conversion on the road to Damascus, Paul took to preaching in the synagogues ... though he had yet to meet with the Apostles (that would come later). He was sort of a
"lone ranger". The upshot of this is that Paul had to be rescued from the wrath of the Jews by the church ... and then sent off to be schooled in the ways of the existing church leadership.
After doing so for 3 years, only then did Paul meet with the Apostles ...