There are some questions that have not been answered. Is this pastor rereading other people's sermons word for word or just borrowing themes and ideas from them? As others have said, ideas get borrowed all the time as can be expected. There is a limited amount of source material, and in 2000 years there is probably very little that has not already been said. I'm sure that most if not all pastors have heard inspiring sermons and later used thoughts and ideas from those sermons in their own sermons.
Second, is the pastor actually claiming that he wrote the sermon. In other words, is his name going on it? If he is attributing someone else's actual work to himself that is plagerism. If it is simply listed in the bulletin as "The Sermon" I'm not sure that qualifies as plagerism. He is not giving attribution, but he also is not actually claiming it to be his own work.