The New Testament uses the words for bishop and elder almost interchangeably, and the office as later understood, a single bishop presiding over the elders and deacons of a city, emerged gradually over the second century rather than appearing complete from the start. Ignatius of Antioch's letters, written early in that century, already argue hard for obedience to a single bishop, which suggests the monarchical model was still being established rather than settled.
Once established, the office spread as the organising unit of the whole ancient church, one bishop to a city, and the historic sees, Rome, Constantinople, Alexandria, Antioch and Jerusalem, carried a seniority that still shapes church structure. The Catholic Church today has several thousand serving bishops worldwide, with the fastest growth in dioceses in Africa and Asia; the Orthodox communion is organised into autocephalous and autonomous churches each with its own bishops; and the Anglican Communion has been openly divided since the 1990s and 2000s between provinces that ordain women and openly gay bishops and provinces, chiefly in the Global South, that reject both, a division formalised in rival international structures such as GAFCON.