Like ancient Israel, early Christians considered worship on the Lord’s Day to be sacred—set apart from the regular, mundane activities of life, and therefore what took place in corporate worship was also sacred. This day was “the Lord’s” in a way different from all other days, and the eucharist was a table belonging to the Lord—“the Lord’s Table”—in a manner distinct from other tables.
The word that emerged to describe the sacred nature of these things was “sacrament.” This term comes from the Latin word sacramentum, which referred to an oath of allegiance, which itself came from the term sacrare, which mean “to consecrate.” Notably, this is the very idea Pliny employs to describe what Christians do when they gather, observing that they meet “to bind themselves by an oath.” This concept fittingly described both baptism and the Lord’s Supper, sacred oaths taken in entrance to and continual communion with the body of Christ.
Likely the first to use the term “sacrament” for both baptism and the Lord’s Supper was Tertullian. He suggested that in the eucharist, the bread represents (Latin, repraesentare) and is the “figuring” (Latin, figurare) Christ’s body. Later in his Latin translation of the Greek NT, Jerome would use the word sacramentum to translate the Greek word mysterion (“mystery”), early Christians considering baptism and the Lord’s Supper to be mysteries, and Augustine (354–430) would later define a sacrament as “the visible form of an invisible grace.”