The word ‘sacrament’, explains Donald Macleod, comes from the Latin sacramentum, which referred to the oath taken by a Roman Soldier. It has therefore been suggested that the Lord’s Supper is the taking of an oath to Christ and an entering into an obligation of loyalty to him. There is, no doubt, truth in this. But the word sacramentum is not a biblical word, and cannot therefore the recruited for theological purposes for a meaning that is not sanctioned by Scripture itself.
More serious, however, is the way in which the word sacramentum entered Christian vocabulary. In the Latin Vulgate it was used to translate the Greek word musterion. Paul in Eph 5:32 refers to marriage as ‘a great mystery’; a great musterion. In the Vulgate marriage become a great sacramentum. One result of this is that marriage came to be seen as a sacrament. Another result was that the close association of musterion and sacramentum meant that the Lord’s Supper came to be seen as a ‘mystery’, in the sense that it is ‘mystifying’.
Based on MacLeod, A Faith to Live By