I cannot tell you how many times I looked at this passage from Mark before I could make some sense of it. The second paragraph is hyperbole on steroids! But what really made me read and pray and wrestle with the text is the last few words, "...and be at peace with one another." That seems to summarize everything that precedes it.
Just before this passage, the disciples were out ministering to people, but they were unable to exorcise a demon. Then, here comes some outsider who successfully exorcises a demon. Look at these words in verse 38, "because he was not following us." There's something wrong with the pronoun us. It says nothing about this man believing in and following Jesus, but the problem was he did not follow the disciples. He was not part of their group! They were jealous! This unknown person did what they could not.
Jesus then continues the dialogue, talking about not putting a stumbling block in the way of "little ones who believe" in him. Aren't we sometimes just a little jealous when we see a church whose parking lot is packed? Isn't it sometimes tempting to speak poorly of another ministry? Can we "be at peace with one another" (v. 50) and give thanks for the way God is using others to help people grow in Christ? There are enough people who need to hear the gospel and need to grow in faith for all of us. Let us especially give thanks for our ecumenical partners and the work we do together in Christ's name.