Orthodox Christianity is a denomination of Christianity whose largest communities exist in Eastern Europe and the Middle East. Orthodox Christianity, like Catholicism, originated with the original Christian Church established by the Twelve Apostles in 30 AD following the crucifixion of Jesus. Its theology is based on the Nicene Creed, adopted at the First Council of Nicaea in 325 AD, and the church was formed by the eastern bishops who disagreed with the Bishop of Rome's claim of primacy over the other bishops at the council. Orthodoxy is characterized by its belief in self-governing churches (as opposed to the Catholic Church, which is centered around the Pope), the belief that Jesus redeemed humanity by triumphing over death at his crucifixion (while Catholics believed that his death itself was a sacrifice made to redeem humanity), praying while standing (while Catholics kneel), the use of leavened bread at rituals (while Catholics use unleavened bread), the ability of priests to marry (unlike the clerical celibacy of Catholicism), and the celebration of Easter according to the Julian calendar (while Catholics use the Gregorian calendar, 13 days ahead of the Julian one). In 1054, in the Great Schism, Orthodoxy broke communion with the Catholic Church, formally splitting, but efforts have been made to rejoin the two denominations due to their many similarities.