Several important scenes in ef take place in a Christian church that is located in the town of Otowa. The church is affiliated with Otobane Academy (a parochial school) and with a Christian orphanage.
Yuu Himura often spends time in the church, to the point where some people mistake him for a priest or a counselor there.
The denomination of the church is never specified in ef. There is evidence that the denomination is likely either Roman Catholic or Eastern Orthodox.
Evidence: Roman CatholicEdit
Because the church contains what appears to be statues of the Virgin Mary, and because the angel Yuuko mentions the Mass, it is possible that the church is Roman Catholic.
Evidence: Eastern OrthodoxEdit
The angel Yuuko wears a box hat and garb that is similar to garments worn by nuns who are Eastern Orthodox. Some of the window illuminations in the church depict bearded saints with crowns, or holding shepherd crooks while hooded, which again are more typical of an Eastern Orthodox church than a Catholic one. (Catholic saints wear miters or are bare headed.) The church and surrounding area is oddly deserted on Christmas Eve day when Yuuko is run over by a car in front of it, and there are no witnesses present to help her before she dies. This might be explained by the fact that in the East Orthodox tradition, Christmas is celebrated not on December 25, but on January 7, and so no one was present.
The Church OrphanageEdit
The great earthquake and fire that destroyed the town of Otowa, Japan, orphaned both Yuuko Amamiya and Yuu Himura when they were young, and they were independently placed in the church orphanage together.
Yuuko was soon adopted by the family of Akira Amamiya. Yuu was never selected for adoption, and he eventually grew up without ever having any adoptive parents. This makes him independent and somewhat bitter.
Ten years later, Mizuki Hayama (Miki) was placed in the same orphanage after her mother committed suicide and killed all the other members of her family. Yuuko and Yuu (who were themselves still minors at the time) became Miki's ersatz parents while she lived at the orphanage. They regularly met with little Miki inside the church, and they cared for her and loved her as much as they could. This might explain why Mizuki grew up with such a well-adjusted and optimistic personality.