Imagine a world where Catholicism was suppressed and vanished in c. 1000. And now imagine that a group of people decide to revive it in c. 2000. So they start wearing medieval clothing, gather at religious feasts where they eat mostly or entirely medieval dishes, refuse to worship saints that were not recognized as such by the medieval Church, structure their beliefs based on the writings of pre-1000 theologians and perform both private and public religious ceremonies that reproduce those of c. 900 – including clothing, replicas of ritual tools and the use of Latin. Compared with modern Catholics, these would-be medieval folks look pretty out of touch, outdated even, if not outright fossilized. And yet their actions are akin to those of some modern-day polytheists.
It’s true that there is a difference. Modern Catholicism is a world apart from its medieval version because it has evolved as a living religion, often by pressure of wider society, whereas Greek, Roman, Egyptian, Norse, Celtic or Canaanite polytheisms were suppressed centuries ago and can only be revived if people connect with the historical expressions of those religions. To that end, you need to immerse yourself in ancient cultures in order to see and experience things the way they did, which in turn allows for a greater understanding of their practices and beliefs. Yet this is not an end in itself; or at least it shouldn’t be. The point of immersion in ancient cultures is to understand what is essential and what is non-essential in the ancient religion you’re striving to revive. What is appearance and what is principle. What is social and what is religious. In other words, the point is to re-emerge carrying basic notions into a modern context. If you stay immersed, you’re not reviving an ancient religion: you’re re-enacting it or, to use Dver‘s words, fetishising an ancient culture.
Of course, this is not to say that you should do away with every trace of the past as if it was outdated trash. That would be as radical as trying to go back in time. Respect and reverence for the ancient world is a good thing, especially when your religion is rooted in it, but, again, one thing is respect and quite another is fetishising or fossilising. Traditional Shinto priests, for instance, wear clothes that are modelled after those of court officials of the Heian period (c.800 – c.1200 CE), which is to say that during ceremonial events, they put on ceremonial robes. Yet most of those simply attending don’t wear medieval clothing and instead use what today is deemed as well-dressed or modern formal wear. In the same manner, Catholics don’t put on medieval clothing while attending mass, though a Catholic priest wears robes whose origins go back several centuries. There’s a difference between conducting the ritual-religious and living in the mundane, the former being more conservative than the latter (especially in orthopraxic religions). And even ritual forms change as social context mutates: ancient Jewish communal worship involved sacrificial offerings, but after the destruction of the second Temple of Jerusalem in 70 CE, it fully morphed into a form that we recognize today. This is an extreme example, granted, but the basic idea stands: context changes living things, including religious ritual, which may remain rooted in the past, but will evolve together with trends, ideas, conditions and social practices. And this is equally true for beliefs.
So Romans conducted their religious ceremony in Latin? Why wouldn’t they? It was their native language! Which means that if you’re interested in reviving their religion, as opposed to simply re-enacting it, you can use your own native tongue in religious ceremonies. Add a few Latin words for a sense of connection with the past, if you will, especially if you don’t speak a romance language, but replicate the principle and not the appearance. Romans used togas during religious events? Of course they did: it was formal clothing at the time! So follow the basic idea by dressing nicely to attend a ceremony instead of imitating the appearance by wearing a toga. Romans never syncretised a Mayan or Japanese god with a Latin one and therefore modern cultores shouldn’t do it either? Ridiculous! The only reason why Romans never did it is because they never came in contact with Mayan or Japanese culture. Unless you want to live in the past instead of reviving an ancient religion in the modern world, you can follow the old practice by syncretising whichever deities you know just as ancient Romans syncretised gods they knew. Singers, poets and political leaders who lived after the 5th century should not be worshiped because doing so would be unhistorical? Again, ridiculous! That’s practising a fossilized religion, not a living one! If Romans honoured heroes and venerable dead known in the ancient world, you can imitate the principle by honouring heroes that lived after the fall of the Roman Empire.
To argue that to use a modern language in a religious ceremony, to worship Jim Morrison as a Bacchic hero and prophet or to syncretise Mercury with Inari is any less genuine or worthy because it is not something the ancient Romans did, is as ridiculous as claiming that modern Catholicism is not genuine because it is different from its c. 900 form. You don’t have to live in a medieval fantasy in order to be an honest, modern Catholic, just as you don’t have to agree with Cicero, take Marcus Aurelius as gospel, speak fluent Latin, wear Roman clothing or stick to what the Romans did in order to be a true, modern Roman polytheist. Unless you want to have a fossilized practice or be the polytheistic equivalent of the Society St. Pius X, you’re free to grow past the historical forms and give new, rooted shapes to an old religion and still be a good cultor or cultrix.