late Old English, in Christian use, "a sacrament of the Church, an outward and visible sign of inward and spiritual grace," especially "the sacrament of the Eucharist" (c. 1300), from Old French sacrament "consecration; mystery" (12c., Modern French sacrement) and directly from Latin sacramentum, "a solemn oath, any engagement or ceremony that binds or imposes obligation," specifically "oath of obedience and fidelity taken by Roman soldiers on enlistment; sum which two parties to a suit first deposit," hence, "a cause, a civil suit;" in Church Latin, "a mystery, a sacrament, something to be kept sacred; the gospel revelation; a Church sacrament" (source also of Spanish sacramento, German Sakrament, etc.), from sacrare "to consecrate" (see sacred). A Church Latin loan-translation of Greek mysterion (see mystery).
In theology, particularly, "a solemn religious ceremony enjoined by Christ, or by the church, for the spiritual benefit of the church or of individual Christians, by which their special relation to him is created or recognized or their obligations to him are renewed and ratified."
The meaning "arcane knowledge; a secret; a mystery; a divine mystery" in English is from late 14c.; from c. 1400, "a solemn oath, pledge, covenant; a ceremony accompanying the taking of an oath or the making of a pledge." The seven sacraments are baptism, penance, confirmation, holy orders, the Eucharist, matrimony, and anointing of the sick (extreme unction).