I think that nowadays people are less acrimonious about religion than some other topics. One way to discuss it is to focus on things that people generally agree on, even if they come from different viewpoints. Another way is to talk about criteria used for evaluating the issues. Another way is to give the different arguments that the different sides make without trying to come down supporting either side.
When you ask why there are so many denominations, you are basically talking about the many Protestant churches that formed after the start of the 16th century Reformation, which typically are Reformed Protestant. That's an important question, and there are different logical ways to answer it, and those reasons relate to basics of the Reformation itself. Indirectly they also relate to much earlier centuries with the development of papal authority, and going back to the very beginning of Christianity, the issue of how authority and institutional unity should work in the Church.
I asked a moderator on a mainstream Presbyterian forum why didn't mainstream Presbysterians unite with Lutherans nowadays, since they have so much goodwill and some common major ideas. From what I recall, the moderator responded to me that the Lutheran Church has a system of bishops whereas the Presbysterian Church has a different system, which relies on elders for church governance.
