Loss and Gain describes the religious climate of Oxford University during the 1840s, a time of great contention between various factions within the Church of England. Some factions advocated Protestant doctrines, renouncing the development of doctrine through tradition and instead emphasising private interpretation of scripture. Against these and other liberal religious factions, the Oxford Movement, of which Newman was a leading member, advocated a Catholic interpretation of the Church of England, claiming that the Church and its traditions were authoritative. Amongst all of these thinkers, however, the Roman Catholic Church was despised as having abdicated its claim to doctrinal authority by introducing superstition into its practice. Accordingly, when Newman converted to Roman Catholicism in 1845, he met with vehement criticism. In Loss and Gain, Newman's first publication after his conversion, he expressed the intellectual and emotional development that led him to Roman Catholicism and the response his conversion elicited. Newman was in his 40s and was an esteemed theologian at the time of his conversion, but in the novel he displaces his experience onto Charles Reding, a young student entering Oxford and experiencing its intellectual climate for the first time. Although Charles attempts to follow a conventional path and avoid being influenced by "parties" (i.e. cliques advocating trendy sectarian views), he soon discovers that he is inclined towards Roman Catholicism. He struggles against this inclination but eventually decides he must convert, a decision that causes great consternation to his family and friends but leads to personal fulfilment.Charles Reding arrives at Oxford University planning to follow the advice and example of his father, and to submit to the teachings of the Church of England without becoming involved in any factious parties. Reding is inclined towards a form of Latitudinarianism, following the maxim "Measure people by what they are, and not by what they are not." His conversations with his friend Sheffield convince him, however, that there must be right and wrong answers in doctrinal matters. To follow the right views, Reding seeks a source of Church authority, and is disappointed to find only party dissension and the Protestant doctrine of Private Judgment, which locates interpretive authority in the individual and thereby leads (in Newman's view) to the espousal of contradictory views. Furthermore, Reding begins to have doubts about the Thirty-nine Articles, to which he must subscribe to take his degree. His doubts are briefly dispelled following the death of his father, but return soon afterward. In particular, several brief encounters with Willis, a former Oxford peer who converted to Roman Catholicism, greatly excite and trouble him. Suspicious of his speculations, Jennings forces Reding to live away from Oxford while studying for his exams, so as not to corrupt other students. Reding confesses his doubts to his sister Mary, who does not understand them and loses trust in her brother. When Reding finally decides he must convert, Mary, his mother, and several family friends express resentment and anger. He travels to London, on the way receiving encouragement from a Catholic priest (perhaps Newman himself), the first he has ever met. While in London Reding is confronted by emissaries from various religious and philosophical sects who, hearing about his departure from the Anglican Church, want to recruit him for their own causes. Ultimately, however, Reding arrives at the Passionists Convent, where he joins the Roman Catholic Church.

Question: What is Reding forced to subscribe to, yet has doubts about?
Answer: Thirty-nine Articles.
Alternative answer: The Church of England