Surveys and Statistical Reports can give a great snapshot on how a specific population (or group of populations) may think, act, or believe at a certain point of time in a certain place or places.
The World Religion Database contains detailed statistics on religious affiliation for every country of the world. It provides source material, including censuses and surveys, as well as best estimates for every religion for the period 1900 to 2050.
Many denominations now post some of their membership statistics online.