Why is Don Abbondio not celebrating the marriage of Lucia and Renzo?

As the local parish priest, Don Abbondio is naturally expected to preside at all church wedding ceremonies. The forthcoming betrothal of Renzo and Lucia is no exception; or at least, it shouldn't be. However, on the eve of the wedding, Don Abbondio is accosted in the street by a couple of thugs as he makes his way home. They tell him in no uncertain terms that he must not go ahead with the wedding ceremony; those are the express orders of Don Rodrigo, a powerful local baron.
Don Abbondio is, among other things, a complete physical coward, and so it doesn't take much persuasion from Don Rodrigo's goons to get him to call the wedding off. Indeed, he's so much of a coward that he can only bring himself to tell Renzo that the wedding has been postponed. But Renzo eventually gets the truth out of the hapless prelate. It turns out that Don Rodrigo wants Lucia for himself.