When a group is considered to form a distinct subgroup within society because its members trace their heritage to a particular country and practice a certain religion, they are thought to be a(n)?

Ethnic group--groups of people who share a common culture including religion, language, and traditions.

In the US, many ethnic groups survive as part of the "melting pot" ideology of US culture. Ethnic groups in the US typically refer to the culture or country of origin as most Americans can trace their ancestry to another place in the world.