The exhibition on Zoroastrianism The Everlasting Flame, currently showing at London's School of Oriental and African Studies, should perhaps be renamed The Ever-Dwindling Flame. There are approximately 137,000 Parsi Zoroastrians left in the world, only 4,100 in the UK (they're outnumbered by heavy metal adherents on the latest census), and for every birth there are five deaths. Possibly the least inclusive community in existence – you can't convert to Zoroastrianism, and it is patrilineal – Zoroastrians need to widen their admission criteria fast, or go the way of the lesser Antillean macaw and the koala lemur.

I say this not because I'm after membership – the Zoroastrian beliefs of my mother's side of the family do not appeal – but because, as the wide-ranging Soas exhibition amply demonstrates, it is a fascinating ancient community that predates even Judaism. Originating in Iran in approximately 1,500BC, founded by the badass apocalyptic prophet Zoroaster, it is the first monotheistic religion, and is based on a simple equation of good (Ahura Mazda) versus evil (Ahriman). Good symbolises creativity, and evil is tantamount to destruction – in the event that there is more evil in the world than good, the planet will be destroyed, so all Zoroastrians must vow to live by the motto "good thoughts, good words, good deeds".

One of the leading world religions for a millennium, Zoroastrianism is best known for its fire temples. Followers believe fire symbolises God's purity and wisdom, so worship in front of a flame. It is also famous for its towers of silence, enclosed spaces where the dead bodies of Zoroastrians are placed to be devoured by vultures. The holy book is called the Avesta, split into two sections (much like the Old and New Testaments, full of stories and myths), and the initiation ritual is named the Navjote ceremony.

As the child of a female Zoroastrian, I was not permitted to follow in my mother's footsteps and undertake the Navjote; whether I am even allowed to call myself Parsi is debatable, although as it is an ethnicity, it is hard to know what else to term myself. And, as a non-Zoroastrian, I am not allowed into the fire temples.

The faith is inextricably linked to the Parsi and Irani communities – you don't find Zoroastrians of any other ethnicity. Although historical evidence suggests that many left ancient Persia for economic reasons, my late maternal grandfather gave me the traditional Parsi version of events. He stated that the Zoroastrians' exclusivity dates back to the seventh century, when Arab Muslims invaded ancient Persia and gave them the choice of converting to Islam or fleeing the country. Many chose the latter, and sailed to India in a fleet of ships. On arrival, the king of the Gujarat sent out a (possibly apocryphal) cup of milk filled to the brim, to signify that the country was overflowing with residents and couldn't accept any more.

The king of the Zoroastrians, however, returned the cup of milk with a spoonful of sugar sprinkled over the top, to indicate that the Zoroastrians wouldn't cause the country to overflow – they would merely sweeten the mixture. The Gujarati king admitted the seafaring refugees on three conditions: that they would promise not to eat beef; not to marry into the existing population; and not to convert any Hindus. The Zoroastrians have stuck rigidly to the latter two tenets ever since, leading to quips like: "We Zoroastrians aren't allowed to convert anyone. So we can go up to people's houses and knock on their doors – but then we have to run away."

Why, as an atheist, should I feel sadness at the decline of a faith that was never mine to begin with? Perhaps for the same reason that many people who don't believe in God have a soft spot for the religion of their ethnicity: nostalgia for my childhood, family ties, the memory of my beloved grandfather praying quietly in his sedreh and kushti. But I think it is more than that: Zoroastrianism and being a Parsi are intertwined, so much so that the terms are often used interchangeably. If the religion dies, all the traditions of the ethnicity will go with it.

I suspect that it will be too late by the time my fellow Parsis wake up to the fact that survival urgently depends on overruling archaic laws. Instead of basing their faith on rules established 1,300 years ago, the Zoroastrians need to adhere instead to the rules of evolution, and adapt or die. For the sake of history and humanity, I hope they choose the former.