“He who is not with Me is against Me, and he who does not gather with Me scatters abroad” (Matthew 12:30).
By this point in His ministry, Jesus is openly healing the sick and performing various miracles; but, rather than being impressed by His power, the wicked Pharisees begin to rationalize and try to attribute this power to Satan.
The designated purpose of Jesus’ performing miracles was to convince witnesses that He was sent from God. With that recognition, Jesus would be able to draw the doubters to Himself and, thus, to God.
Since the Pharisees were refusing to be convinced, Jesus levies this firm statement of fact: they were either with Him or they were working against Him. There was no middle ground.
Nothing about the reality of this statement has changed since that day: it is just as true today that if a person is teaching doctrine contrary to the doctrine Jesus taught or if he or she is living in such a way that doesn’t reflect Jesus’ example, he isn’t really with Him.