Short answers, pulled from the story.
In ecclesiology, the Christian Church refers to what different Christian denominations conceive of as the true body of Christians or the original institution established by Jesus Christ. The term is also used in academic contexts as a synonym for Christianity as a whole, even though it comprises many distinct denominations.
The English word church derives from the Old English "cirice," which came from West Germanic "kirika," itself from the Greek "kuriake," meaning "of the Lord." Most Romance and Celtic languages instead use forms of the Latin "ecclesia," which traces back to the Greek ἐκκλησία, meaning an assembly or congregation.
On the 27th of February 380, Emperor Theodosius I decreed that the Nicene version of Christianity was the official religion of the Roman Empire. The decree classified all other forms of Christianity as heresy, which was made illegal, and in 385 Priscillian became the first person executed under this new legal order.
The Great Schism of 1054 was the formal split between the Western Catholic Church, centered in Rome, and the Eastern Orthodox Church, centered in Constantinople. The rupture grew out of long-standing political and theological tensions, culminating when Rome and the East excommunicated each other in the 11th century.
The Four Marks of the Church, first expressed in the Nicene Creed in 381, are that the Church is one, holy, catholic (universal), and apostolic, meaning originating from the apostles. Different Christian traditions interpret these four marks differently when making competing claims of legitimacy.
The Catholic Church teaches that it is the original institution founded by Christ on the Apostles in the 1st century AD. The Second Vatican Council's Lumen gentium, issued in 1964, declared that the one Church of Christ subsists in the Catholic Church, governed by the successor of Peter and the bishops in communion with him.