The religion of the Romans was different in a number of profound ways, not just from modern Christianity, but from the very ways in which we today tend to think about the purpose, function, and characteristics of religion. The most obvious difference is that Roman paganism was a polytheistic religion, meaning that there were many gods. For the Romans, the world was a place inhabited by an infinite number of gods, including many that they had not heard of. When the Romans encountered other religions, they were very open about adding these new gods to the list of those they already worshiped. Thus the Roman pantheon was constantly expanding due to the addition of new gods. This attitude is vividly illustrated by a ritual called the evocatio. This occurred when the Romans were about to attack and possibly destroy an enemy city. Before launching the assault, the Roman priests would formally invite the gods of the city to abandon it and take up residence and be worshiped at Rome,
The pantheon of Roman gods included deities who resembled humans {such as Jupiter), personifications of abstract qualities (such as Victoria, the personification of victory), nature spirits or deities usually associated with geographic places or bodies of water (such as Father Tiber), and a variety of gods imported from foreign cultures (such as the Egyptian goddess Isis). Each individual would pick one or more gods to worship as his or her particular guardians. Since certain gods were associated with specific cities and professions, these gods would probably have received particular attention from people of that profession or who lived in that city.
Roman religion did not possess a standardized sacred text like the Bible. While there were certain rituals, such as sacrifice, that were commonly prescribed for worshiping the gods, there was no central and all-encompassing theology. Roman religion was a loose collection of diverse gods and practices allowing a great deal of variety and personal choice. There were hundreds—perhaps thousands—of divinities that were worshiped in Roman polytheism. Further adding to the complexity of Roman religion was the fact that there were different types of gods whose powers, inclinations, and areas of influence varied greatly. The state religion was based on worshiping a subset of all the gods who were thought to be particularly concerned with the success and preservation of the Roman state, but individuals could choose any combination of gods to pay homage to.
The most prominent Roman gods were what might be termed the Olympian gods. This set of deities, derived from the Greek gods said to live on Mount Olympus, included Jupiter, Juno, Mars, Venus, Neptune, Apollo, Diana, Ceres, Bacchus, Mercury, Minerva, Vesta, and Vulcan. The most important of these for the Romans were Jupiter, the king of the gods, and Mars, the god of war, both of whom were thought to be especially interested in the success of Rome.
Often, however, these major gods were multiplied through the addition of epithets that identified some particular aspect of the god. These epithets were usually related to either a location or an activity. For example, there were Jupiter Capitolinus (the Jupiter who lived on the Capitoline hill) and Mars Ultor (Mars the Avenger), to whom Augustus dedicated a temple in commemoration of his avenging the assassination of his adoptive father, Julius Caesar. Jupiter alone had at least 19 different epithets.
In addition to these gods, there were what might be called demigods, who were often men who had attained divine status, such as the Greek hero Hercules and Romulus, the founder of Rome. There were many entities that might be called gods as well, such as spirits of streams, rivers, and trees. Such a god was a genius loci, literally "the spirit of the place." Some gods were personifications of abstract qualities. The most important of these to the Romans were Fortuna, or luck, and Victoria, victory. Finally, there were all the gods borrowed from other cultures, including Egyptian, Etruscan, and Germanic ones. The Romans were extremely open to adopting new gods that they encountered and adding them to their pantheon. Further complicating Roman religion was the fact that, when encountering new foreign gods, the Romans sometimes decided that these gods were simply local variants of gods they already knew.
Thus, it is almost deceptive to speak of a single notion of godhood in Roman culture since there was such a variety of forms that divine beings or spirits could take. Nor did they fit into any clear hierarchy. Any attempt to create such a hierarchy would quickly run into contradictions and problems in logic; even to try to do so is a modern concept and something that the Romans themselves never attempted.