The idea that a pastor or elder is unchallengeable flies in the face of the Lordship of Christ alone. Of course we may be challenged.
I think Pastor Reuther handles the question very well. If we are to be leading by example, shouldn't we be the first to be open to 'challenge'. It's of course very helpful when the challenge comes with the requisite tact and humility. And I do think 'dialogue' is a better term. I welcome nothing more than serious engaging conversations about my teaching and my actions -- I need to be sharpened, too, and I'm encouraged to know that my ministry is significant to their thought and life. Like everyone else, I can get my hackles up -- but it's usually a problem on my end when that happens (e.g., I realize deep down that I'm wrong but am slow to admit it). And if I don't happen to be in the wrong, I would much prefer the opportunity to persuade a dissenter than to have dissent just fermenting in the background.
Now if by challenge, something more is meant, such as accusation, then it should be brought by two or more, per 1 Tim. 5:19.
Thanks for the post.