What is the proper etiquette for a non-Catholic who attends Mass?
Non-Catholics should sit and stand when Catholics do the same. If their convictions do not allow them to stand, then they may remain seated. Non-Catholics do not have to kneel, although it would be more fitting if they did. When it is time for Communion, Non-Catholics should remain seated while Catholics line up. It is highly inappropriate for a non-Catholic to attempt to receive the Eucharist.