Mark 9:39 Commentary - Whedon's Commentary on the Old and New Testaments

39. That can lightly speak evil of me If he uses my name he must believe that mine is a divine name. If he can work wonders by it, his faith must be both true and great. If God enable him to perform miracles, the divine obligation is upon him. He therefore cannot speak contemptuously of the name by faith in which he achieves miracles of mercy. He win never pronounce I am an impostor, or join the scribes in saying that I cast out devils through the prince of devils.

It cannot be denied that this furnishes a strong reproof to bigots who are ready to deny the Christian or the churchly name to those who are not of their own organization. Where those who differ from us do not endanger the fundamentals of repentance towards God and faith in the Lord Jesus Christ, we must be very cautious how we forbid them because they follow not us. We may love our own doctrines, discipline, usages, and denomination, we may defend them at the proper time with firmness and boldness; but let us not be unjust to the piety and the zeal in the cause of Christ and the good of mankind, which exist in other denominations of Christians.