A well-established religious organization in which a substantial number of the population is a member.

Answer: Denomination

Denominations can be found in all the religions. As long as a group of belives is recognized by the church they can have their own identity and preach accrding to their will. If the group is not recognizes by the church we have something that is called a cult, and that is on the opposite spectrum from denominations. In the US, there are a lot of Christian denominations because the constitution states the free will to practice religion. In Europe there are only few denomination within the Chatolic and Ortodox church. For example, Presbitarians would be denomination whereas Scientology is a cult.