The Yamaha Sound Reinforcement Manual is an excellent introduction and technical reference.

Sound reinforcement in a reverberant space like a church is a nightmare. It's typically done badly "to save money", and sometime done again properly, at which point the money spent on the first system has been wasted. Unless you are lucky you need someone with a lot of knowledge and experience to get it right.

This is a church which was served by a succession of retired ministers, each of whom had his regular Sunday. Many of them were over 80. They would stand at the front of the pews and deliver sermons to all of us, including people 80 and 90. In three years, I never heard a complaint that they couldn't be heard.

Really, that's your entirely practical, tried-and-tested answer!

If the church is very reverberant, sound absorbing material, correctly chosen and applied, may be what you need more than clever electronics.