breaking your styles up into timeperiods *may* be a place to start (that is after IDing your styles to show research and analysis). Then, look at what may have influenced these "changes" eg Vatican 2, move /increase/rise in Pentecostalism, ppl/numbers attending church etc (these eg's can be linked but *probably* are not the best for linking-does that make sense?)
Also, have to decide what you "mean" by Christian. ie are you just doing Catholic or all/a particular Protestant church, are you comparing styles between variants etc. as this will also dictate direction and research.
Also, you * may* want to look at how is (and how was) the "music" used ie in liturgy,masses, formal and informal, live music Vs recorded, "audience" participation, use at particular times of the Mass (if doing Catholic- as music serves specific purposes during the MAss- ask your music teacher about this one! etc
Apparently, there is some really good stuff on the net for this- if I can find a music ISP student, I'll ask...... (ours have their ISP's back so this shouldnt be a problem)
Hopes this isnt too confusing (am really stuffed and tired after planning and running 6 Ash Wednesday services at school today!
SH