'Sacrament' was a Roman military oath repurposed by Christians to translate Greek 'mysterion.'
A religious rite or ceremony regarded as an outward and visible sign of inward and spiritual grace; in Christianity, traditionally referring to baptism, the Eucharist, and (in Catholic and Orthodox traditions) five additional rites.
From Latin 'sacrāmentum' (an oath, a pledge, a military oath of allegiance, a deposit as guarantee of good faith in a lawsuit), from 'sacrāre' (to make holy, to consecrate), from 'sacer' (holy, sacred, set apart). The PIE root is *sak- (to make a compact, to sanctify). Roman soldiers swore the 'sacrāmentum' — an oath to the standards, to the general, to Rome
The city of Sacramento, California, takes its name from the Sacramento River, which Spanish explorers named 'Río del Santísimo Sacramento' (River of the Most Holy Sacrament) in the seventeenth century. The capital of the most populous U.S. state is thus literally named after a religious rite rooted in Roman military oaths.