‘But Jesus said to him, “Do not forbid him, for he who is not against you is for you.” '
But Jesus told them that they were wrong to forbid him. For if this man was being successful then it demonstrated that God was with him, and that he believed in Jesus. And such a man should therefore be encouraged, for he clearly did support them even though he was not with them. Had he not truly believed in Jesus his exorcisms would not have been successful (as the disciples had cause to know could happen).
‘He who is not against you is for you.' Jesus is here speaking of those who were active in proclaiming the word of God and in ministry of that word, not just of anyone in general. If men truly ministered the word and were not antagonistic to Jesus and His disciples then they were clearly on the same side together. For had they not been they would have been antagonistic.
The question is sometimes posed, ‘would a man have cast out evil spirits in Jesus name while Jesus was still alive?' The answer is undoubtedly, ‘yes'. There were many potential exorcists around in Jesus' day, and they would always be alert to anything that could make them more successful. If they saw the disciples successfully casting out evil spirits in Jesus' name (Luke 10:17) it is certain that some would see that as another name that they could experiment with. It is probable therefore that quite a few commenced using the name of Jesus, as did the sons of Sceva after them (Acts 19:13-14). The point here is that the man used it successfully. And Jesus' point is that His Father would not have given success were his belief in Jesus not genuine. He was thus in his heart ‘one of them'.