Brideshead Revisited

Although he aspires to become an artist, middle-class Charles Ryder (Matthew Goode) reads history at the University of Oxford, where he befriends the flamboyant and wealthy Lord Sebastian Flyte (Ben Whishaw). Sebastian's mother, Lady Marchmain, strongly disapproves of his life-style, especially his heavy drinking. When Sebastian takes him home to visit his nanny, Charles is enthralled by the grandeur of the Marchmain family estate, known as Brideshead, and he is entranced by its residents, including the devout Roman Catholic Lady Marchmain (Emma Thompson) and her other children, Sebastian's elder brother Bridey (Ed Stoppard) and his sisters Julia (Hayley Atwell) and Cordelia (Felicity Jones).
When Lord Marchmain invites Sebastian and Julia to visit him and his mistress Cara (Greta Scacchi) in Venice, Lady Marchmain encourages Charles to go with them in the hope that he can act as a positive influence on her son. Increasingly interested in Julia, Charles surreptitiously kisses her in a dark alley, unaware that Sebastian can see them from the other side of a canal. Jealous of his attention to his sister, Sebastian sets out to end their friendship, and on their return to England Lady Marchmain makes it clear that Charles cannot marry Julia since he professes to be an atheist.
Sebastian's mother, concerned about his increasing alcoholism, cancels his allowance. During a visit to Brideshead, Ryder gives Sebastian money, which he uses to buy alcohol. When he arrives drunk and improperly dressed at a ball celebrating Julia's engagement to Canadian business man Rex Mottram (Jonathan Cake), Lady Marchmain blames Charles and tells him he no longer is welcome at Brideshead. Sebastian flees to Morocco. Some time later, at the request of the now terminally-ill Lady Marchmain, Charles goes there and finds Sebastian in hospital. His plan to bring Sebastian home is disrupted when the doctor warns him Sebastian is too ill to travel.
Years later, Charles â now married and a successful painter â is reunited with Julia on an ocean liner travelling to England from New York. They realise they are still in love and decide to leave their respective spouses and live together. Charles returns to Brideshead, where he persuades Rex to step aside so he and Julia can be together. Rex agrees to the proposal in exchange for two of Charles's paintings, an arrangement that shocks and angers Julia, who feels like bartered goods. Without warning, a dying Lord Marchmain and Cara arrive at Brideshead so he can spend his final days in his former home. On his deathbed Lord Marchmain, hitherto an atheist, regains his faith and dies reconciled to the Roman Catholic Church. Deeply affected by her father's transformation, Julia decides she cannot relinquish her own faith to marry Charles, and the two part.
Several years later, during the Second World War, Charles â now a disillusioned army captain â finds himself once again at Brideshead, which the army has requisitioned as a military base. A corporal tells him Julia is serving in the reserves and that her elder brother died during the Blitz. In the movie's final scene, Charles visits the family chapel, where he finds a single lit candle. He dips his hand into holy water and moves to snuff out the candle between his fingers, but leaves the flame still burning.

Question: Why does Lady Marchmain make it clear that Charles cannot marry Julia?
Answer: Charles professes to be an atheist.