CAIRO — The Dec. 11 bombing of a church in the Cairo cathedral complex — the seat of the Coptic pope — has been claimed by the Islamic State, although the Egyptian government has blamed the Muslim Brotherhood. Whoever planted the bomb that killed 27 people, including a 10-year-old girl, when it ripped through a church full of Sunday worshipers understood well how endemic bigotry in Egypt has left Christian lives at the whim of a regime that pays lip service to protecting them, armed Islamists who actively seek them harm, and a public that largely does not care.

To gauge the enormity of what happened at the church of St. Peter and St. Paul, imagine a bombing in a church within the Vatican complex. Egyptian churches have been bombed before, but this was the first time a bomb had been taken inside a church to directly target worshipers; and it was the first time that Islamic State affiliates in Egypt targeted civilians after months of attacking the police and, in the Sinai province, the military.

Coptic Christians are the largest Christian community in the Middle East, numbering about 10 percent of Egypt’s 90 million people. The cruel reality for Egyptian Christians is that only the Egyptian military has killed more Christians in recent times than did the Dec. 11 bombing. In October 2011, 28 Christians were killed in clashes with the military outside the Maspero television building in Cairo. They were protesting against an attack on a church.

At the time, Abdel Fattah el-Sisi was the head of military intelligence and a member of the Supreme Council of the Armed Forces, which ruled Egypt after President Hosni Mubarak stepped down following the Jan. 25, 2011, revolution. Mr. Sisi has portrayed himself as the savior and protector of Egyptian Christians because it was he who ousted President Mohamed Morsi of the Muslim Brotherhood and has led the effort against the Islamist insurgency in Sinai. In 2013, Mr. Sisi became the first Egyptian president to attend Christmas Mass, and this month he attended the funeral of those killed in the Dec. 11 bombing. But Christians remain vulnerable to both the regime and its armed militant opponents.