If you are part of a tiny sect it matters not at all, the others in your new group are happy to have anyone. If you are part of a very large sect then there is a need to divide into smaller groups because a person can only have so many friends and caste is an the fallback to divide on.
Most people have family of other religion, and so they often do something about family. Sometimes that means other-cast friends in the religion are kept apart from family, sometimes it means they are thrown in the face of family as a way to show how different they are.
I know more about Christian practice than Muslims. I suspect the same logic applies but I'm not sure.
If you are part of a tiny sect it matters not at all, the others in your new group are happy to have anyone. If you are part of a very large sect then there is a need to divide into smaller groups because a person can only have so many friends and caste is an the fallback to divide on.
Most people have family of other religion, and so they often do something about family. Sometimes that means other-cast friends in the religion are kept apart from family, sometimes it means they are thrown in the face of family as a way to show how different they are.
I know more about Christian practice than Muslims. I suspect the same logic applies but I'm not sure.