It is evident that the Marchmain family followed Catholicism due to family obligation and in particular due to Lady Marchmain prizing duty above all else. To paraphrase Cordelia’s words, in the Marchmain family, Catholicism was synonymous to Lady Marchmain. This is illustrated by Lord Marchmain’s conversion to Catholicism in order to please Lady Marchmain. It is also illustrated by Sebastian who at the onset of the show describes himself as a half-heathen owing to his complex relationship with Lady Marchmain. Sebastian is portrayed as only adhering to the Catholic faith out of duty. Additionally, he remarks that he finds the ideas of the Catholic faith to be lovely which to some extent appears to suggest that his belief in Catholicism is only due to aesthetics. Ultimately, Sebastian illustrates the conflict between Catholic aristocratic tradition on the one hand and the evolving secular society on the other. In the end, he is unable to reconcile the two warring ideas and consequently he runs away to a North African convent where he settles down as a religious drunk.
In addition to adhering to the Catholic faith out of family obligation, majority of the characters in the show seem to think that there is virtue in suffering and that only through suffering will they be drawn closer to God. Cordelia while discussing Sebastian with Charles remarks that no one can ever be holy without suffering. Her own sacrifice is shown when she transforms from a lively character into a pious and pompous one who has lost all her prior enthusiasm. Arguably, it is also the reason why Julia decides to leave Charles after her father’s death as she felt that it was the only way that she could be returned to God. I would thus posit that the understanding of religion within the Marchmain family is not only out of family obligation but is also due to the belief that to be holy is to suffer.
For Rex, religion was used as a tool that gave him a better standing within the society. The case is different with Charles. A self proclaimed agnostic, Charles is puzzled by the constant presence of religion in the everyday life of the Marchmains. Instead, Charles substitutes religion with a love for aesthetics. Indeed, aesthetics becomes his own theology as is illustrated by his tendency to ascribe an aesthetic value to everything including his affection for Julia. It is only when he watched Lord Marchmain accept the Last Sacrament before his death that he began to long for a sign. When the audience is reintroduced to Charles a few years later, it is made clear that his conversion to Catholicism has not brought him any relief. Only at the end, when he watches the red flame at the chapel does he find hope.
I do not agree with the view of religion portrayed in Brideshead Revisited and instead posit that the central case of religion is natural law. Aquinas argues that the fundamental principle of natural law is to do good and avoid evil. He further states that human beings are naturally inclined to know that there are a variety of things that are intrinsically good and which should be pursued such as procreation. Additionally, he holds that human beings not only know what is good but are able to distinguish through reason, which actions are morally right in pursuing those goods. 
However, an analysis of Aquinas work shows that regardless of the fact that human beings are aware of the precepts of natural law, they cannot account for how the guiding principles of natural law come into force. Natural law thus manifests the existence of a more than human-source that is the creator of natural law.  The recognition of a Supernatural Being as intelligent, benevolent and the provider of all things enables human beings to perceive their own reality and everything that they rely on as a gift and consequently, realize that the creator of natural law guides them on how to act within a world whose very existence depends on Him. 
In conclusion, natural law is the inherent factor in human beings that enables them to be guided to do good and avoid evil. Natural law is also the intrinsic element in human beings that enables them to apprehend the existence of a Supernatural Being. This is the reason why I agree with Aquinas supposition that natural law is the central case of religion.
 Murphy, Mark, “The Natural Law Tradition in Ethics,” The Stanford Encyclopedia of Philosophy (Winter 2011 edition). Available at https://plato.stanford.edu/cgi-bin/encyclopedia/archinfo.cgi?entry=natural-law-ethics
 Grisez, Germain, “Natural Law, God, Religion and Human Fulfilment”, The American Journal of Jurisprudence 46, no.1 (2001):3-36 available at http://www.nlnrac.org/contemporary/new-natural-law-theory/primary-source-documents/Natural-Law-God-Religion-and-Human-Fulfillment