This is why one of the best preachers I ever heard (from a technical delivery standpoint-not talking about his message in this forum) taught that it was best to use a handheld mic-not a lavalier or headset. He also railed on sound guys that used an automatic compressor or manually "compressed" him-but he was an expert at handling the mic and controlled his own volume incredibly well-he could anticipate the changes which neither a sound person or an automatic compressor can do. IMO, that is still the best way-and if someone is going to being a preacher/speaker they should learn to use the tools of the trade just as any master craftsman should. But we have developed a culture that values quick and easy over mastering a craft/trade.
Of course, the readers of this forums job is to support the preacher/speaker/artist the best we can whatever their skill level.
Best answer yet ^^^^
I served in audio ministry several years mostly Baptist or Pentecostal preachers in a sanctuary that seated maybe 350. The preachers usually start out whispering and end up full voice singing/tuning the sermon and inhaling heavily by the time they deliver the landing (hooping for those who know what I'm talking about). The more novice ops would always have that slight feedback from overdoing the gain in the beginning of the message when the preacher was whispering. Many of those preachers wanted loud monitors too which was perfect recipe for feedback. The op could have just let the volume be low - almost too low if you got hammered by a loud praise band/choir prior to the sermon.
As the sermon progressed, I usually end up easing down the mains gain almost to nothing or else the system would definitely go to clipping unless there was brick wall limiting. You have to keep in mind the preacher is preaching. This is not a lecture where the delivery is the same from start to finish. Sometimes by the time the sermon reaches that final point, the only thing left coming from the PA was just monitors although the congregation was hearing every word at decent volume. Again, the more novice ops were never afraid to turn up in the beginning but were always hesitant to turn down during peak. The other thing was the more novice guys always would be hesitant to high-pass the preacher mic.
If the volume varies greatly in the santuary, thats fine. The dynamics are part of the delivery. I'm more apt to put compression/limiting on the recording and broadcast channels than the live mix though.