All Roman soldiers, at the beginning of any military campaign, took an oath of allegiance.
The usual method was that one soldier in each legion was called forward by its tribune and asked if he would swear that he would obey the commands of his generals and execute them promptly, that he would not desert, and that, in battle, he would not leave the ranks except to save the life of a Roman citizen.
After that oath was taken, each of the other soldiers, in answering to his name, then said, “Idem in me (The same for me).”
This oath was termed sacramentum, literally, an action of sacred nature. Its violation by any soldier was sufficient cause for the general to order his death without trial.
The term was also used in courts of law, applying to the sum of money deposited by the contestants of a suit.
But perhaps because of the formal nature attending the ceremony of sacrarnentum, the term was taken by Christian writers of the third and later centuries to be the equivalent of the Greek mysterion, a term originally applied to the secret rites attending the worship of Greek gods, but in Christian use applied to the rites connected with Christian worship.
The term sacrament continues to apply to those rites observed in Roman Catholic and Protestant churches, whereas mystery is the term for similar rites in the Greek Orthodox Church. (Compare MYSTERY.)