Other forms: congregations
Although the word is most usually assigned to the members of a church, any gathering might be called a congregation, including a gathering of animals. Come to think of it, a congregation of church members is often called a "flock."
The Latin root of congregation, which is greg, meaning "flock," easily becomes congregare, meaning "to gather together," and finally congregationen, giving the current meaning of “a group.” The meaning took on a religious quality when 16th Century Protestants took it to refer to the church itself, then refining it to mean church members. If you're not a churchgoer, you might still refer to your friends who show up to watch football as a congregation.