1. Ancient Roman Law: The word "sacramentum" first appeared in Roman law, where it referred to a sacred oath or promise. This oath was made by both parties involved in a legal dispute, and it was often accompanied by a deposit of money. If one party broke the oath, they would lose the deposit.
2. Early Christianity: Christians adopted the word "sacramentum" to refer to sacred oaths and promises made to God. This was particularly used in the context of baptism and the Eucharist.
3. Augustine of Hippo: While the use of "sacramentum" in early Christianity is established, Augustine of Hippo (354-430 AD) was instrumental in developing a more theological understanding of the term. Augustine defined a sacrament as an outward sign of an inward grace, laying the groundwork for its future use in Christian theology.
Therefore, while we can't name a single individual who first used the word "sacramentum," its origins lie in Roman law and its adaptation into Christian theology was greatly influenced by figures like Augustine of Hippo.