We forbade him - See this subject considered on Mark 9:38 (note), etc.

(10) And John answered and said, Master, we saw one casting out devils in thy name; and we forbad him, because he followeth not with us.
(10) Extraordinary things are neither rashly to be allowed nor condemned.

And John answered and said, Master,.... The Syriac and Persic versions read, "our Master":
we saw one casting out devils in thy name, and we forbad him; See Gill on Mark 9:38.
Because he followeth not with us; the Syriac, Persic, and Ethiopic versions read, "because he followeth not thee with us"; did not join in company with them, and follow Christ along with them, and as they did.

John answered, &c.--The link of connection here with the foregoing context lies in the words "in My name" (Luke 9:48). "Oh, as to that," said John, young, warm, but not sufficiently apprehending Christ's teaching in these things, "we saw one casting out devils in Thy name, and we forbade him: Were we wrong?" "Ye were wrong." "But we did because he followeth not us,'" "No matter. For (1) There is no man which shall do a miracle in My name that can lightly [soon] speak evil of Me' [Mark 9:39]. And (2) If such a person cannot be supposed to be 'against us,' you are to consider him 'for us.'" Two principles of immense importance. Christ does not say this man should not have followed "with them," but simply teaches how he was to be regarded though he did not--as a reverer of His name and a promoter of His cause. Surely this condemns not only those horrible attempts by force to shut up all within one visible pale of discipleship, which have deluged Christendom with blood in Christ's name, but the same spirit in its milder form of proud ecclesiastic scowl upon all who "after the form which they call a sect (as the word signifies, Acts 24:14), do so worship the God of their fathers." Visible unity in Christ's Church is devoutly to be sought, but this is not the way to it. See the noble spirit of Moses (Numbers 11:24-29).

Mark 9:38.