Answer:
A religious group is a group of people that share a belief system in one or more gods with their own rituals. Different cultures can share the same religion even if they don't speak the same language, eat the same food or even live in the same part of the earth.
An ethnic group is a group of people who are connected with their shared culture and beliefs that were passed down from one generation to another for a long time.
A good example of an ethnic group can be a Greek person living in America. He and his family come from Greece and they are American but their ethnic background is Greek. Where religion can be practiced by many ethnic groups but apart from religion they don't have much else in common.