The word simply means a "substitute." Thus the adjective is frequently used in biblical theology to denote the "vicarious death of Jesus," i.e., of His substitutionary suffering and death on our behalf.
The noun used in the context of the Christian church denotes a person who holds the authority to serve as the substitute of another, e.g., the bishop when he is away on duty elsewhere. In the Church of England, the title denotes a minor clergy such as the parson of a parish (who receives the smaller tithe or salary). The office of Vicar Apostolic is conferred on a person charged by the Pope of Rome to serve in some distant post which has no bishop or to substitute for one who was, for various reasons, unable to discharge his duty. Another office of the Roman Catholic Church is that of Vicar General; it is sometimes used of the pope himself (see Vicar of Christ below) but more often it is applied to an appointee, perhaps an archdeacon, who serves in place of the bishop. This was the title carried by Thomas Cromwell when he served under Henry VIII.
The title Vicar of Christ is one that popes of the Catholic Church have claimed for their office since the 11th Cent when Western Europe was mired in a struggle for power with the monarchs, the question of who—the Pope or the Monarch—has the final say in matters concerning the nation, and especially concerning the Church. This was most clearly evident in the Investiture Controversy, over the question of whether the monarch may present to a newly appointed senior bishop with the symbols of sacrad office, or whether that honour belonged to the Pope alone. Pope Gregory VII (r.1073-85) seemed to have won that round. His successor, Pope went one further when, in he declared the Pope of Rome the Vicar of Christ. The Pope was not merely the 'Vicar of Peter,' a title the popes of Rome had always claimed for themsleves; what claimed for his office was to be Christ's substitute and representative on earth, whose "duty was to lead the task of making the world and the Church holy". The declaration was as bare-fisted an attempt at spiritual bullying as there ever was one. The history of the papacy has, of course, since then proven how hollow was the claim, even if some popes may have privately entertained the sentiments that that was what they were meant to do.
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