The Sacramentum was a military oath taken by all Roman legion

Roman legion

A Roman legion normally indicates the basic ancient Roman army unit recruited specifically from Roman citizens. The organization of legions varied greatly over time but they were typically composed of perhaps 5,000 soldiers, divided into maniples and later into "cohorts"...

aries on entering the Roman army

Roman army

The Roman army is the generic term for the terrestrial armed forces deployed by the kingdom of Rome , the Roman Republic , the Roman Empire and its successor, the Byzantine empire...

, part of the state ritual created by Augustus

Augustus

Augustus ;23 September 63 BC – 19 August AD 14) is considered the first emperor of the Roman Empire, which he ruled alone from 27 BC until his death in 14 AD.The dates of his rule are contemporary dates; Augustus lived under two calendars, the Roman Republican until 45 BC, and the Julian...

 during his military reforms in the early 1st century CE. By the 3rd century it was administered annually, on 3 January, as attested by the calendar of state ritual discovered at Dura-Europos

Dura-Europos

Dura-Europos , also spelled Dura-Europus, was a Hellenistic, Parthian and Roman border city built on an escarpment 90 m above the right bank of the Euphrates river. It is located near the village of Salhiyé, in today's Syria....

, the so-called Feriale Duranum, which dates to the reign of Severus Alexander (222-235 CE). At the same time a new altar to Jupiter Optimus Maximus was erected.

In the rare case of punishment by decimation

Decimation (Roman Army)

Decimation |ten]]") was a form of military discipline used by officers in the Roman Army to punish mutinous or cowardly soldiers. The word decimation is derived from Latin meaning "removal of a tenth".-Procedure:...

, the surviving legionaries were often required to renew this military

Military

A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

 oath

Oath

An oath is either a statement of fact or a promise calling upon something or someone that the oath maker considers sacred, usually God, as a witness to the binding nature of the promise or the truth of the statement of fact. To swear is to take an oath, to make a solemn vow...

 which was part of the state religio at the foundation of Roman military discipline.

At the start of the 3rd century Tertullian

Tertullian

Quintus Septimius Florens Tertullianus, anglicised as Tertullian , was a prolific early Christian author from Carthage in the Roman province of Africa. He is the first Christian author to produce an extensive corpus of Latin Christian literature. He also was a notable early Christian apologist and...

, in De corona, condemned any Christian soldier's willingness to swear the sacramentum, since baptism

Baptism

In Christianity, baptism is for the majority the rite of admission , almost invariably with the use of water, into the Christian Church generally and also membership of a particular church tradition...

 was the only sacrament

Sacrament

A sacrament is a sacred rite recognized as of particular importance and significance. There are various views on the existence and meaning of such rites.-General definitions and terms:...

a Christian should observe.