In ancient Roman religion and law, the sacramentum was an oath or vow that rendered the swearer sacer, "given to the gods," in the negative sense if he violated it.
The sacramentum legis actio was a sum of money deposited in a legal procedure[5] to affirm that both parties to the litigation were acting in good faith.
The text of the oath was recorded by Vegetius:[8] The sacramentum that renders the soldier sacer helps explain why he was subjected to harsher penalties, such as execution and corporal punishment, that were considered inappropriate for civilian citizens, at least under the Republic.
[10] In the rare case of punishment by decimation, the surviving legionaries were often required to renew their oath, affirming the role of state religio as the foundation of Roman military discipline.
By the 3rd century the sacramentum was administered annually, on January 3, as attested by the calendar of state ritual discovered at Dura-Europos, the so-called Feriale Duranum, which dates to the reign of Severus Alexander (222-235 AD).