John Calvin Commentary Mark 9:39

"But Jesus said, Forbid him not: for there is no man who shall do a mighty work in my name, and be able quickly to speak evil of me." — Mark 9:39 (ASV)

Forbid him not. Christ did not wish that he should be forbidden; not that he had given him authority, or approved of what he did, or even wished his disciples to approve of it, but because, when by any occurrence God is glorified, we ought to bear with it and rejoice. Thus Paul (Philippians 1:18), though he disapproves of the dispositions of those who used the Gospel as a pretense for aggrandizing themselves, yet rejoices that by this occurrence the glory of Christ is advanced.

We must also attend to the reason which is added: that it is impossible for any man who works miracles in the name of Christ to speak evil of Christ, and therefore this ought to be reckoned as gain. For from this it follows that if the disciples had not been more devoted to their own glory than anxious and desirous to promote the glory of their master, they would not have been offended when they saw that glory heightened and enlarged in another direction. And yet Christ declares that we ought to reckon as friends those who are not open enemies.