What about Byzantine Empire? Did they have any pagan communities after ~500 AD?
A famous visual example of "pagan" practices post-313 is the Diptych of the Nicomachi/Symmachi, which indicates that pagan practices were still strong in Rome around the turn of the 5th century.
(Scroll down the V&A link to read "The Pagan revival at the end of the 4th century" at the bottom...or if you are lazy, here's an excerpt:
"At the end of the 4th century in Rome there was a desperate last-ditch battle by the wealthy patrician families to preserve the old pagan religions, the ancient way of life, in the face of the ever-growing strength of Christianity, which had become the official state religion in 380.
The Symmachi and Nicomachi families were at the centre of the resistance and did as much as they could to protect the old beliefs against the antagonism of the Christians. They paid for the upkeep of the temples out of their own pockets, debated with the Christians about the merits of tolerance in religious matters, and edited ancient texts such as Livy and Vergil for the benefit of future generations.")
For a surviving text of Vergil, see something like the Vatican Vergil
hi! not discouraging anyone from chipping in on this, but FYI, you might find these related posts of interest
Excluding the Vikings, when did the last pagans disappear from Western Europe?
How did Lithuania hold out against christianization for so long?
How long did it take to convert pagan Icelanders and Greenlanders to Christianity?
Why was Sweden the last of the Scandinavian countries to convert to Christianity?
According to A Chronicle of the Last Pagans (Pierre Chuvin), the Laconians in Greece were not converted by the Byzantines until the 9th Century. That may have been the last "community" with clear roots back to pre-Christian times, but paganism may have survived in other forms, disguised as academic philosophy or masquerading as other, more tolerated religions. For instance, the Sabians of Harran had a murky religious background that lasted until the 11th Century, and the philosopher Plethon was pushing pagan religious ideals as late as the 15th Century.
I had wanted someone else to write a top-level answer on this, as I haven't read the most recent literature on this (most importantly Alan Cameron's The Last Pagans of Rome), but there are a few misconceptions here that I'd like to address.
Whilst the facts stated here are all correct, that Julian was the last pagan emperor, that the Serapeum was destroyed by a Christian mob (probably) and that some Senatorial families fought to defend paganism, recent scholarship has moved away from viewing Later Roman paganism as diametrically opposed to Christianity. Instead, it is better to see them as uneasily co-existing, with aspects of both cultures found on both sides, or more radically, to see the paganism as a dying force well before Theodosius and the 'pagan reaction' of the late fourth century as a political movement that used paganism as a vague symbol rather than something they passionately believed in (which is I believe Alan Cameron's argument). Personally, I'm more inclined to think that paganism was still relevant to people's lives even in the fifth century, but I do agree that the degree of pagan reaction/Christian persecution has been overstated - the transformation of the empire to a Christian state was a slow one and most pagans took it in their strides.
Examples of conflict for instance can always be seen the other way. Julian did try to restore paganism, yet it is clear from people like the rhetor Libanius that many pagans weren't convinced by him, as Julian himself tried to reform pagan beliefs into something that can challenge Christianity, which actually annoyed a lot of conservative pagans. In any case, Julian's successors, Valentinian and Valens, though themselves Christians, generally had bigger issues to worry about than persecuting pagans - their purge of the Senatorial class (for their use of magic) is now generally seen as a purge of their political enemies, rather than a way to persecute pagans. Plus, persecuting magic-users was a very Roman thing to do, so what they did wasn't unusual. Their successors were more complicated. Gratian in the West was by all account a very devout emperor, to the extent of refusing the title of Pontifex Maximus and removing the Altar of Victory from the Senate House. This annoyed pagans no doubt, but his actions, particularly removing the Altar, weren't that unprecedented - Constantius II had already removed the Altar a few decades earlier (it was restored by Julian and left in place by Valentinian I). Gratian was soon murdered by a usurper, and the new legitimate emperor was Valentinian II, a mere boy, and the pagans quickly sent a petition to the capital to urge the restoration of the Altar/public funds for pagan institutions. This is often seen as an attempt by the pagans to take advantage of a weak administration, yet the men who were involved in this, Symmachus and Praetextatus, were already in important offices before this and their careers were never hindered by the many pro-Christian laws passed by Christian emperors. Instead, it is better to see the petition as the result of both their faith and personal interests, as aspects of the petition reads more like a mish-mash of demands from different people, rather than a coherent manifesto of resistance. Perhaps more significantly, this petition was published by Symmachus' son to increase the prestige of his family decades later, which suggests that Symmachus' actions weren't all that dangerous, since even in the Christian world of the fifth century, a rebuke of imperial policy can still be published for public consumption!
The state did issue laws that persecuted pagans, but they were often ineffective in reality. There was for instance a law issued by Valentinian I that banned Manichees from gathering in Rome, but a few years later the Manichee Augustine arrived in Rome and found a thriving Manichee community, his connections being so successful that he was appointed as a court orator in Milan soon afterwards. In 407, the now Christian Augustine wrote a letter to the pagan leader of a nearby African town where a riot between Christians and pagans took place, urging all parties to calm down and not to involve the imperial government in their dispute. The same was true for the burning of the Serapeum and later the lynching of the philosopher Hypatia in the fifth century - the pagans were the losers in these confrontations, but these events were not planned, but spontaneous results of increased social tensions and political intrigue. The riot that led to the burning of the Serapeum for example started because some pagans held a parade with their religious symbols around Alexandria, which angered the Christian bishop. Anyway, rioting was common in the Later Roman/Byzantine Empire and happened for all sorts of reasons, so although paganism played a part, we can't just ignore other factors at play. Whilst it wasn't all fun and games to be a pagan, we shouldn't take harsh legislation or riots against pagans at face value - the very fact that laws and riots were instigated against pagans would suggest that a lot of pagans were doing quite well for themselves!
Lastly, whilst Symmachus was involved with the Nicomachi, despite their family connections and their shared faith Symmachus did not join in the revolt of Eugenius - he had already been implicated with an earlier usurper and he probably didn't want to risk the wrath of Theodosius again. Eugenius was also a Christian usurper and he had plenty of political reasons to try to get the pagan Nicomachi on his side, as they were one of the richest families in Rome, so the idea of it being a 'pagan last-stand' is inaccurate. Symmachus was soon back in favour after Eugenius' death and his son's marriage to the daughter of the elder Nicomachus was not seen as a bad match in retrospect, being celebrated in a fifth-century inscription in Rome. The Roman elite, despite their religious differences, still shared a common culture, one that encompassed both Christianity and paganism, rather than separating them into two different cultural worlds. This is why in the fifth century we can find pagans like Claudian writing Christian panegyrics for the imperial court, or Volusianus as an imperial ambassador to Constantinople. Christians likewise were happy to absorb pagan learning and deal with pagans as political friends or enemies depending on the circumstances. Public Classical paganism died a slow and gradual death, which makes its decline rather unexceptional. In private of course people still believed in whatever they wanted, especially in the undocumented countryside, and as you can see from my answer on pagans in the Byzantine Empire, we can still find pagans in high places nearly two hundred years after Constantine!
The best I think you're going to get for a "last stand" for paganism is the emperor Julian the Apostate, the last pagan emperor of Rome. Julian was a convert from Christianity, and he tried hard during his reign to revive paganism and give it new life. Paganism would live on long after Julian, but his reign was really that last concerted effort for paganism to reestablish its former dominance, after him, it was a long and slow decline.
Another candidate for "last stand of the pagans" is the destruction of the Serapeum in Alexandria Basically Theodosius I, the emperor at the time, outlawed paganism throughout the empire. Pagans retreated to the Serapeum, one of the last functioning temples in the empire, were besieged by a mob of Christians, and the temple was destroyed.