A religious community is a community (group of people) who practice the same religion.
In the wider sense it may describe members of one religion who live near or intermingled with members of other religions, who may often mix in everyday life but worship separately. One might speak of the Catholic community of Derry (a city), or the Jewish community of France (a country). The community may be defined informally: people who practice, say, Catholicism consider themselves members of the Catholic community of their region. In other cases the distinction is more formal. To become a part of a Baptist church, you need to be accepted as a member. And in Israel only people from the same officially recognized religious community may marry each other (see marriage in Israel).
In a narrower sense a religious community is a group of people living together specifically for religious purposes, such as in a monastery.