One such example was Bacchus (Dionysus): Why make sacrifices when you could celebrate with debauched feasts and violent carnal orgies which culminated in rape? The lude cult had to be put down by the authorities and as well as a series of imprisonments it is said that a number of persons committed suicide as a result of the public embarrassment. If nothing else this tells us that there was a degree of morality and moral standards within society powerful enough to drive one to suicide.
Clearly the case of Bacchus was an extreme one, but it makes the point. One less extreme example was the goddess Magna Mater – Cibele – whom according to the advice of the soothsayers the Romans duly uprooted and imported to Rome from the East. A ship was dispatched to fetch the statue of the goddess and import her through the port of Ostia up the Tiber in great pomp and ceremony. Finding the eunuchs to run the priestly order was a problem but the Senate solved it by using foreigners. A sign of some degree of moral limit?
The truth is that as with everything else, Rome became a melting pot of all the traditions and influences of the lands it invaded. Slaves for example would be brought to Rome from far afield and they, little by little, acquired the liberty to meet up and follow their own religions so long as they didn’t go against Rome and its institutions. These were the “novensiles“. Respected gods but the gods of captives nonetheless. These gods had to find lodgings in temples built with private funds but they weren’t outlawed by any means.
This was very much the way with Christianity which by the end of the empire came to dominate over other religions. However, the problem with Christianity was that like Judaeism it was a monotheist religion and therefore forbade its followers from paying the due religious respects to emperor-gods such as Nero or Diocletian. This automatically broke the “not against the Roman state” bit of the freedom of worship rule and made its followers into anti-Roman traitors.
Oriental religions were particularly successful, particularly with the Roman matrons, and gave rise to great temples such as that of the Egyptian god Isis. There was a very famous temple to Isis near the Pantheon of which the huge bronze pine cone (symbol of longevity) stands in the Vatican gardens. Another interesting religion which was possibly imported from Persia but then exported by Roman soldiers all over the empire was Mithraism.
Mithraism is heavily steeped in mystery but on the basis of the evidence collected seems to have a cosmological explanation as there are repeated images of stars, the bull (Taurus), the twins (Gemini), a dog, scorpion and snake, all of which are to be found as constellations. One recent theory relates this to the discovery of precession and the movement through the constellations of the sun at the date of the spring equinox. We can quickly imagine a sun-god able to move the heavens about.
Mithraism is regarded as the primary religion competing with Christianity towards the end of the empire until the declaration of Christianity as state religion brought about almost immediate destruction of all Mithraic temples and altars. A number of these have been brought back to light (in fact they tended to be in under ground caves). One such temple and altar is found under the foundations of the church of San Clemente in the centre of Rome. In fact there are several Mithraeums under Rome but the one at San Clemente is particularly poignant.
A curious anecdote is that some reports suggest that some parts of the Mithraic liturgy were similar to those of the Christian (or vice versa), including the words of some of the prayers.