For he who is not against us is for us. He does not enjoin us to give a loose rein to rash men, and to be silent while they intermeddle with this and the other matter, according to their own fancy, and disturb the whole order of the Church: for such licentiousness, so far as our calling allows, must be restrained. He only affirms that they act improperly, who unseasonably prevent the kingdom of God from being advanced by any means whatever. And yet he does not acknowledge as his disciples, or reckon as belonging to his flock, those who hold an intermediate place between enemies and friends, but means that,. so far as they do no harm, they are useful and profitable: for it is a proverbial saying, which reminds us that we ought not to raise a quarrel till we are constrained.

He that is not against us, is on our part - Or rather, Whosoever is not against You, is for You. Instead of ἡμων, us, I would read ὑμων, you, on the authority of ADSHV, upwards of forty others, Syriac, Armenian, Persic, Coptic, Ethiopic, Gothic, Slavonic, Vulgate, Itala, Victor, and Opt. This reading is more consistent with the context - He followed not us - well, he is not against You; and he who is not against you, in such a work, may be fairly presumed to be on your side.
There is a parallel case to this mentioned in Numbers 11:26-29, which, for the elucidation of this passage, I will transcribe. "The Spirit rested upon Eldad and Medad, and they prophesied in the camp. And there ran a young man, and told Moses, and said, Eldad and Medad do prophesy in the camp. And Joshua, the servant of Moses, said, My lord Moses, forbid them! And Moses said unto him, Enviest Thou for My sake? Would God, that all the Lord's people were prophets, and that the Lord would put his Spirit upon them." The reader will easily observe that Joshua and John were of the same bigoted spirit; and that Jesus and Moses acted from the spirit of candour and benevolence. See the notes on Numbers 11:25-29 (note).

For he that is not against us, is on our part. Many copies read, "he that is not against you, is for you"; as this man; he was not against either Christ, or his disciples; he was doing the same work, promoting the same interest, and destroying the kingdom of Satan: and therefore, though he did not follow them, and had not his commission immediately from Christ; yet, inasmuch as he was opposing the same common enemy, and did nothing against them, he ought to be reckoned as one for them, and on their side. It is a proverbial expression, signifying that all that are not against a man, and take not the part of his enemy, are to be accounted his friends.

For he that is not against us is on our part--Two principles of immense importance are here laid down: "First, No one will readily speak evil of Me who has the faith to do a miracle in My name; and second, If such a person cannot be supposed to be against us, ye are to consider him for us." Let it be carefully observed that our Lord does not say this man should not have "followed them," nor yet that it was indifferent whether he did or not; but simply teaches how such a person was to be regarded, although he did not--namely, as a reverer of His name and a promoter of His cause.

For he that is not against you, is for you - Our Lord had formerly said, he that is not with me, is against me: thereby admonishing his hearers, that the war between him and Satan admitted of no neutrality, and that those who were indifferent to him now, would finally be treated as enemies. But here in another view, he uses a very different proverb; directing his followers to judge of men's characters in the most candid manner; and charitably to hope that those who did not oppose his cause wished well to it. Upon the whole, we are to be rigorous in judging ourselves, and candid in judging each other.