Pulpit plagiarism is a phenomenon in which a pastor takes a sermon originally by another preacher and re-casts it in some way or another for his own congregation without informing them of this fact. I've conversed with a number of people about this topic. Recently someone directed me to the definition of plagiarism found on John Piper's website (link to definition - article by Matt Perman).
Perman seems to think that plagiarism, where it is intentional, is stealing. I'm not sure that's necessarily the case, though I see the point that Perman is making. Let's apply that issue to the situation of sermons. Is it plagiarism if a pastor essentially borrows another sermon he heard on-line without crediting the original author?
What if the pastor reads a great sermon by one of the Puritans and decides to make it his own by updating the language and shortening it by 50%? What if he does the same thing with the sermon of a living pastor?
Finally, how should we Christians react to discovery that a pastor has been engaging in plagiarism? Is this is a Matthew 18 situation? Are we required to keep our knowledge of the fact that something is plagiarism a secret until we have attempted to provoke repentance on the part of the preacher?