If you are writing the history of a whole denomination you will need to select representative individuals and congregations to use as illustrations. It will be necessary for you to know the overall story well so that you can see the things which are common to a number of individual situations. When you come to describe that general factor you can then illustrate it with one or two examples. Include some personal reminiscences. Describe how people coped at different periods.
Small details can bring things to life.
Statistical information is valuable, but not always easy to collect or to interpret. Different churches have their own categories of membership and some of us are better at recording this sort of information than others. Try and gather as much information as possible. If you are able to chart the growth of your church from its beginnings that is always helpful. For tracing growth in membership, if the figures are available you will need to note what definition was used in gathering the figures.