Sacramentum is the Latin translation of the Aramaic raz and the Greek μυστήριον. It is this term that the English term 'Sacrament' is derived. Sacramentum by its origin and nature was - unlike raz and μυστήριον NOT properly defined as a religious term. It was in fact a secular term and this is critical.
Lewis ans Short identify the origin of the term as "the sum which the two parties to a suit at first deposited, but afterwards became bound for, with the tresviri capitales; so called because the sum deposited by the losing party was used for religious purposes, esp. for the sacra publica; v. Fest. p. 344."
In the military the term initially was used to describe the preliminary engagement entered into by newly-enlisted troops (this was followed by the proper military oath, jusjurandum, which was at first voluntary, but, after the second Punic war, was demanded by the military tribune).
It eventually acquired a religious color later but the term sacramentum first of all was a legal term concerned with Roman military discipline. Hence the phrase Sacramentum militiae. By this it meant, the oath of loyalty which every new recruit had to make at his entrance into the Roman army.
This is only in late Latin that the term came to mean "something to be kept sacred" in various forms i.e.
1. A secret: “sacramentum regis abscondere,” Vulg. Tob. 12, 7.—
2. The gospel revelation: nolite verba, cum sacramentum meum Erit canendum, providenter quaerere, Prud. στεφ. 10,15.—
3. A mystery: “sacramentum stellarum,” Vulg. Apoc. 1, 20: “pietatis,” id. 1 Tim. 3, 16; Lact. 7, 24; Aug. de Agone Christi, 24.—
4. A sacrament: “signa, cum ad res divinas pertinent, sacramenta appellantur,” Aug. Ep. 138: “baptismi,” id. Doctr. Christ. 3, 13: “sanguinis Christi,” id. Ep. ad Bonif. 98, 9: “(matrimonii),” Vulg. Eph. 5, 32.—
5. The office of the ministry: “Athanasium episcopum... coctus in unum quaesitus (synodus ut appellant) removit a sacramento quod obtinebat,” Amm. 15, 7, 7.
The real difficulty we should have with the adoption of THIS PARTICULAR TERM in favor of μυστήριον is that Latin already had adopted the Greek term as mysterium. The question we have to ask ourselves is how a term originally used to for military discipline by Emperors and generals could have NATURALLY been chosen to replace the Greek word when a Latin equivalent already existed.
The fact that this occurred in the late second century and early to mid third centuries where Irenaeus and countless other pieces of evidences point out that the Imperial government was having an active role in developing and 'reforming' Christianity had to have factored into this development. After all one could argue that the the Libros quinque adversus haereses and all variants had as their sole purpose spying out disloyalty to the proscribed 'oaths' of Roman baptism.
