Introduction
The religion of the early Roman soldier was not distinguishable from the religion of the average citizen, as Republican soldiers were recruits or conscripts who returned to civilian life after their tour of duty. By the time of the empire the legions had become a professional fighting force, the chief appendage of the ever-expansive Roman State. The religion of the imperial legions was therefore modeled on the State religion. While there is no doubt that many if not most of the soldiers sincerely believed in their divinities, the chief political effect of the army religion was to promote Romanization among the troops. This was especially important as Italians deserted the legions and provincials came to form the backbone of Roman military power.
Christianity
While there probably had always been Christians serving in the legions after the mid first century, the number of open Christians seem few and far between. Christianity was not quite compatible with the State religion, which no doubt encouraged some Christians to keep their faith hidden. Moreso, however, many of the legion recruits came from rural and backwater areas that were the last reaches of the empire to experience conversion to Christianity. It has been said the Roman army was the last refuge of paganism. The traces of Christianity found in the imperial legions are slim. The rest of this essay will concentrate on paganism.
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The Standards of the Legion
Armies throughout history have found it vital to promote an esprit de corps, a sense of pride and belonging to one's unit. In the imperial legions this took on a quasi-religious element which is perhaps unique even within paganism. The various cohorts within the legion held their own standards, and the legion itself, since the times of Marius, was represented by an eagle. The fearsome and elegant bird of pray was a perfect symbol of the legion's fighting prowess; more so it was the sacred sigil of Jupiter, King of the Gods and patron of the Roman State.
The Eagle was made of gold or silvergilt. It was mounted on a large pole with a tapered point at the end which could be thrust in the ground. On the pole beneath the eagle were imagines, medallions featuring portraits of the emperors. The legion's eagle, along with the cohort standards, were housed within a small shrine in the legion's headquarters. The first cohort of the legion and in particular the primus pilus was charged with guarding the eagle, though the day-to-day care was in the hands of the aquilifer. The aquila was considered sacred and oaths were sworn by it. The legion felt the aquila embodied their spirit and dignity; it was a kind of genius for the company of soldiers. The standards were utilized in various festivals and ceremonies, in particular the anniversary of a legion's founding. To lose the aquila was the ultimate disgrace, and battles were fought merely to regain lost aquila.
Christianity often made baseless accusations of "idol worship" against paganism. However, there may have been some truth to it with regard to the peculiar cult of the legion standards.
Gods of the State
Jupiter, Juno and Minerva were a holy trinity constituting the Capitoline Triad. These trio of gods had been the patron divinities of the Roman Republic for some five centuries, and the sheer weight of tradition continued into the empire even when the Republic was a distant memory. Jupiter the Best and the Greatest was held in particular esteem. He was considered the special protector of the Roman people. In Republican times he was the god of victorious generals. Votive inscriptions and offerings to Jupiter and his two feminine counterparts were quite common in military installations.
Mars, the ancient god of war and mythological father of the Roman people, was also a divinity near and dear to a soldier's heart. In imperial times Mars had been recast as the divine avenger of the Caesars, and troops marching to war from Rome would proceed from the temple of Mars.
Hercules, the demigod of strength and protection, was honored by some.
Needless to say the soldiers honored the imperial genius of past and present emperors.
In addition to these proper gods, a variety of personified divine traits were also honored. Victory, Discipline, Fortune, Honor, Piety, Virtue all had their cults. The city of Rome was also personified and worshipped alongside the imperial genius.
Gods of Private Cults
Outside of the State religion existed an ever increasing number of private cults, most often composed or inspired by the cultures of Rome's subject peoples. Unlike the State religion they did not have as their central tenant loyalty to Rome and its gods. Nonetheless most of these cults were tolerated as they posed no overt threat to the social order. In some cases they were even encouraged if they promoted ideals useful to the social order. From the military standpoint there were two private cults of significance.
Jupiter Dolichenus was a god of Asia Minor and Syria identified with Jupiter. He was apparently a deity associated with iron and the military might that flowed from it. Jupiter Dolichenus was depicted as an axe-wielding warrior mounted on a bull, and much like Jupiter was capable of flinging lightening bolts. His community of believers called each other "brothers." Dolichenus was an example of a cult that was tolerated but not encouraged. It's rituals were Oriental, its clergy were Oriental, and the majority of its adherents were soldiers from Oriental origins. In short, it was fundamentally foreign if benign.
Far more successful and acceptable to the Latin West was the cult of Mithras. Mithras may or may not have been descended from a very ancient Indo-Iranian solar deity, but by the time the cult of Mithras came to Rome he is a Hellenistic savior god with astrological overtones. Mithras himself was depicted as a soldier, invincible and implacable in battle, the slayer of demons. He promised strength and success to his followers in this world, and a special afterlife in the next world.
His cult met in private for secret rites about which little is known. What is known is that the cult was comprised of imperial officials. Soldiers of all ranks could belong, but it seems officers were more plentiful than rank and file. Also included were officials in the civil service, as well as prominent traders and craftsmen, and the educated slaves of the imperial household. In short, Mithraism was a cult for middle ranking members of imperial society. While Romanatis was not strictly speaking the point of the cult, in practice it seems the cult was fiercely loyal to the personas and values of the imperial order. The members of the cult were also fiercely loyal to each other. The opportunity for these officials to "network" may have been as strong a factor in recruitment as any purely religious sentiment. Contrary to popular belief there is little evidence for the inclusion of emperors or senators; however, the ruling elite may very well have promoted the cult from behind the scenes given its loyalty.
Mithraism was a "foreign" cult that in many ways exemplified the spirit of Rome and its soldiers.
Other Gods
As Rome was composed of soldiers of many cultures, these soldiers often honored the gods of their native cultures in their off-duty. However, throughout the empire there was a tendency to identify Roman gods with native ones, especially in the West.
Soldiers, whatever their nationality, often honored gods of whatever region they were stationed. Soldiers in Britain made offerings to the genius of the province. Soldiers in lower Egypt honored a crocodile god that seems to have been formerly worshipped by Egyptian soldiers. Other examples abound and are too numerous to mention.
Conclusions
The imperial military was arguably the section of Roman society most devoted to Roman paganism, and certainly it was the element of society where paganism survived the longest. In one sense, the devotion to the Empire's deities was another manifestation of a soldier's duty and service to Rome. In another sense, the cultivation of favors with numerous deities was thought to assist soldiers in the constant dangers they faced in their profession, and in some cases was thought to ensure a sound afterlife should they fall in battle.
The Roman legions faced innumerable dangers and brought Western Civilization on their backs to new lands. Their travails were sustained in no small sense by their sense of religious piety. Regardless of what we moderns think of their particular religions, we can at least honor what their religions meant to these intrepid warriors.
Did you know...
Just as it was for the civilian population, religion was a vital part of Roman army life.