In Roman thinking, harmony in human affairs depended on concord between the gods and men and was achieved by the performance of religious rites, of which the most important were solemn offerings. Religion and spiritual aspirations were often the inspiration of Roman art. Reliefs relating to the cult of a god were very common, for instance, particularly in imperial times. This was partly because they were less expensive to produce than sculptures in the round, partly because they were better suited to narrating the legends associated with the god and could be combined with symbolic or secondary motifs. In the general quest for a higher, transcendent realm, Roman art developed its own architectural models, which differ from Greek antecedents in a feeling for space, larger dimensions, illusionistic decoration, lack of feeling for sculptural detail and the importance ascribed to the façade. Masters of the world following the Punic Wars, and seen as favoured by the gods, the Romans sought to create an art corresponding to their god-given vocation. Having glorified the gods and goddesses of eternal life and love — Venus, Diana and Bacchus — they went on to exalt the emperor as bringer of salvation. It would, of course, be inappropriate to see all Roman art as having a sacred function, but it is evident that the allegorical representation of a transcendent world is a frequent aspect of it. Sarcophagi bespeak the aspiration of the Roman soul, and the symbolic value of the scenes they depict helps explain the changes made by the Romans to the Greek myths.

Even portraiture exhibits a religious approach to life: in the third century, for example, the new way of representing the eyes indicates a change in the relationship between the individual and the gods.

Through sculpture, it is also possible to study the penetration of foreign cults into the Empire: the worship of Egyptian divinities introduced from Alexandria in the last two centuries before Christ and the mystery religions, particularly that of Mithras. Deriving from oriental prototypes in which the god is associated with a bull, Mithras was ofren depicted in reliefs of various kinds. The one in the Louvre, originally from Fiano Romano, shows the banquet of Mithras and the Sun, before a cylindrical altar with an undulating snake. The carcass of the bull is laid out in front of them. But not all such monuments illustrate the same text or doctrine, even when they are inspired by a known myth. Modes of representation vary according to tradition, region and period.

Religious observance could be public or private, and the State religion was propagated by the ritually ordained layout of the city itself. Sacrifice was one of the basic religious practices: ritual sacrifices performed on fixed days, sacrifices of an expiatory or propitiatory nature, and thanksgivings. They might involve the death of animals, or a modest libation. Rich and poor alike solemnly performed these rites, advertising the fact within their family circle and the public at large. The ceremonies were frequently marked by the kind of processions shown on Roman reliefs. The altar in the vestibule of the Vatican Belvedere expresses the dynastic programme of Augustus, with the apotheosis of Caesar, or more probably Agrippa, which prefigures his own, Victory bearing a shield, and Augustus himself delivering the Latin to the magistri. Another altar in the Vatican Museum, which includes female subjects, illustrates the social policy of the Emperor, who sought to involve all groups in acclaiming different aspects of the State.

In the Roman world, a multitude of altars were set up in honour of emperors, in addition to those dedicated to the Lares and those associated with the worship of gods. Any study of sculpted altars must take into account the differences between cult altars and votive pillars, altars dedicated to gods and those used to invoke the divinities for funerary purposes. Votive altars are themselves an offering and are often simple in their decoration. From art of the most official kind (Ara Pacis, Arch of Titus, statues of emperors as gods) to the most private and varied manifestations of faith (Source of the Seine and Mont Sacon sanctuaries) and the many statues of gods remarkable fur their original form or a particular private interpretation, religion and the use made of it by those in power was undoubtedly the driving force behind Roman art, albeit with many variations.