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Sacraments › … › II. The Sacrament of Holy Orders in the Economy of Salvation
The priesthood of the Old Covenant
The chosen people was constituted by God as "a kingdom of priests and a holy nation." But within the people of Israel, God chose one of the twelve tribes, that of Levi, and set it apart for liturgical service; God himself is its inheritance. A special rite consecrated the beginnings of the priesthood of the Old Covenant. The priests are "appointed to act on behalf of men in relation to God, to offer gifts and sacrifices for sins."
Source: Catechism of the Catholic Church (Vatican.va)