For he that is not against us is on our part.

Wesley's Notes

9:40 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.

Jamieson-Fausset-Brown Bible Commentary

40. 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.

Matthew Henry's Concise Commentary

9:30-40 The time of Christ's suffering drew nigh. Had he been delivered into the hands of devils, and they had done this, it had not been so strange; but that men should thus shamefully treat the Son of man, who came to redeem and save them, is wonderful. Still observe that when Christ spake of his death, he always spake of his resurrection, which took the reproach of it from himself, and should have taken the grief of it from his disciples. Many remain ignorant because they are ashamed to inquire. Alas! that while the Saviour teaches so plainly the things which belong to his love and grace, men are so blinded that they understand not his sayings. We shall be called to account about our discourses, and to account for our disputes, especially about being greater than others. Those who are most humble and self-denying, most resemble Christ, and shall be most tenderly owned by him. This Jesus taught them by a sign; whoever shall receive one like this child, receives me. Many have been like the disciples, ready to silence men who have success in preaching to sinners repentance in Christ's name, because they follow not with them. Our Lord blamed the apostles, reminding them that he who wrought miracles in his name would not be likely to hurt his cause. If sinners are brought to repent, to believe in the Saviour, and to live sober, righteous, and godly lives, we then see that the Lord works by the preacher.