But Jesus said to him, "Do not hinder him; for he who is not against you is for you." (Luke 9:50 NASB)
But Jesus said, "Do not hinder him, for there is no one who will perform a miracle in My name, and be able soon afterward to speak evil of Me. (Mark 9:39 NASB
The account of the disciples' rebuke of the man who was casting out demons in the name of Jesus is also included in the gospel of Mark, where we find additional detail. The disciples had seen a man casting out demons (an area of recent personal failure for them) in the name of Jesus. He was not known to the disciples and not part of their group and they sought to restrain him.
Jesus, however, had other ideas. If he is not against you, he is for you. He was performing miracles in the name of Jesus and no other. Although he wasn't part of their "group" he was no less on the side of Christ. The decision about who was "for" Jesus did not rest with the disciples, nor does it rest with us.
Jesus made another point that is very interesting. No one can perform a miracle in the name of Jesus and soon afterward speak evil of Him. That statement is worth pondering. The commentaries I consulted had little to say about this, but it's worth considering. The worth translated as "miracle" is dynamis and indicates the power to perform the miracle. The word translated as "be able to" is dynamai and indicates power to perform a task. Both words come from the same root. When we have the power to perform a miracle in the name of Jesus, we will not also have the power to speak evil of Christ afterward.
If a reported miracle worker can speak evil of Christ after performing said miracle, we should not trust the miracle. What is amazing is that, when the power of God flows through us for the performing of miracles, it has a restraining power over our tongue, so that we cannot speak evil of (or curse) our Lord.
It is always the power of God that brings cleansing, never our own actions. Unworthy though we be to act in the name of Jesus, when the power of God flows through us, it should purify our mouths and make us not only more like Jesus but also make our words more glorifying to Him. Perhaps this concept could be better understood as the fact that a "clean" tongue (speech that glorifies God) should accompany our service to Him, especially if signs and wonders, or miracles, are performed in the name of Jesus. Our words, if injudicious, can completely undermine our service to God. In fact, our words are often the standard by which people judge the body of Christ and our place in it. James spoke of this in powerful terms:
But no one can tame the tongue; it is a restless evil and full of deadly poison. With it we bless our Lord and Father, and with it we curse men, who have been made in the likeness of God; from the same mouth come both blessing and cursing. My brethren, these things ought not to be this way. (James 3:8-10 NASB)
The words we say matter. They matter as they bless or harm our Lord, and they matter as they bless or harm our witness. We would do well to consider what our words say about the source of our power. Do our words demonstrate the power of God at work in our lives or not? Those tongues of ours are like a deadly poison, and only God can purify them. Let's invite our Lord to make those words pure and sweet, speaking only blessings and not curses. Create in us, Lord, not just a clean heart but also a clean mouth.