To fully understand the part a religion plays in the life of an individual or a community, I would suggest that we need to examine it from several different points of view, only some of which can be addressed by observing a visible religious activity.
Using Ninian Smart's 'dimensional model' as a starting point, we can examine the different dimensions of a religion and discover how much insight into each of them the observation of a visible religious activity would allow, and consider other means of gathering the information needed to gain a full and rounded understanding of the role of a religion in the life of a community or individual.
It seems fairly clear that the practical and ritual dimension would be well covered by the observing of visible religious activity, though perhaps not by observing just one religious activity. Even within one religion, such as Christianity, the visible activities range from baptism to funeral rites, with all kinds of annual commemorative festivals, such as Christmas, Michaelmas and Easter, along with daily and weekly rituals, such as church services, private prayer and confession thrown in for good measure.
To observe any one of these rituals, each of which has its own unique place in the life of the individual or
community, would leave us with a far from complete understanding of the Christian religion as a whole. Even one ritual can have huge variation within the same religion. Baptism, for example, can range from sprinkling a few drops of water on a baby's head, to the full body immersion of adults, depending on which particular branch of Christianity you look at. So, even here, I would suggest, that observation of a single visible religious
activity offers us a far from complete picture of the part a religion plays...