Why do some people stir up opposition while claiming to serve the gospel?
According to the context, some individuals become so obnoxious, seemingly for the sake of the gospel, that they instigate opposition. Their conduct, not necessarily the gospel itself, provokes resistance. Paul warns the Corinthians not to equate such suffering with proof of belief, nor to shame them, but to treat them as beloved sons in the faith.
Les explains that these things happen when people literally become so obnoxious, seemingly for the sake of the gospel, that they instigate this kind of opposition. Paul then warns the Corinthians that he is not rehearsing these things to make them think that if they have not suffered like he has, they are not believers. He says he does not write these things to shame them, but as his beloved sons. He calls these pagan Corinthians sons in the faith the same way he called Timothy his son in the faith, not physically, because he brought them the gospel and the salvation power that brought them out of paganism and idolatry. In verse 15, Paul says that though they have 10,000 instructors in Christ, they do not have many fathers. Les notes that a person can have many Sunday school teachers, pastors, and television evangelists, but probably only one or two really influenced their life to become a child of God.