Complex sentence: Some traditions maintain that there are certain circumstances under which individuals should not present for communion, and that those invdividuals should voluntarily refrain from receiving communion; however, were those individuals to present for communion, they would not be denied.
Split into simpler sentences: Some traditions maintain that there are certain circumstances under which individuals should not present themselves for (and should voluntarily refrain from receiving) communion. However, if those individuals were to present themselves for communion, they would not be denied.