In Orders or in Holy Orders. Belonging to the clerical order or rank.
The word “order” means not only a mandate, but also an official rank, and in the Catholic Church, a “rule” of life, as Ordo albus (white friars or Augustines), Ordo niger (black friars or Dominicans). In “Holy Orders” is in the plural number, because in the Protestant Church there are three ranks of clergymen—deacons, priests, and bishops. In the Catholic Church there are four major orders and four minor ones. According to Du Cange, the Ordines majores are Subdeaconatus, Deaconatus, Presbyteratus, and Episcopalis (Subdeacon, Deacon, Priest, and Bishop).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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