Commentaries:
Barnes' Notes
Forbid him not - Do not prevent his doing good. If he can work a miracle in my name, it is sufficient proof of attachment to me, and he should not be prevented.
Can lightly speak evil of me - The word here rendered "lightly" means quickly or "immediately." The meaning of the passage is, that he to whom God gave the power of working a miracle, by that gave evidence that he could not be found among the enemies of Jesus. He ought not, therefore, to be prevented from doing it. There is no reason to think here that John had any improper designs in opposing the man. He thought that it was evidence that he could not be right, because he did not join them and follow the Saviour. Our Lord taught him differently. He opposed no one who gave evidence that he loved him. Wherever he might be or whatever his work, yet, if he did it in the name of Jesus and with the approbation of God, it was evidence sufficient that he was right. Christians should rejoice in good done by their brethren of any denomination. There are men calling themselves Christians who seem to look with doubt and suspicion on all that is done by those who do not walk with them. They undervalue their labors, and attempt to lessen the evidences of their success and to diminish their influence. True likeness to the Saviour would lead us to rejoice in all the good accomplished. by whomsoever it may be done - to rejoice that the kingdom of Christ is advanced, whether by a Presbyterian, an Episcopalian, a Baptist, or a Methodist. Compare Philippians 1:18.
Other Barnes' Notes entries containing Mark 9:39:
Matthew 18:1-6
Mark 9:42-50
John 3:26
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