The Middle East is full of ancient, mysterious religious sects. The Islamic State is wiping them out.

When I met Mirza, a Yazidi nursing a leg he broke trying to rescue his people from the brutal Islamic State, I was fascinated by his religion. The Yazidis believe that nothing is outside the mercy of God. Even Lucifer, they think, has been redeemed and restored to favor; his tears of repentance extinguished the fires of Hell forever.

I’ve spent the past four years writing a book about the mystery religions of the Middle East — ancient versions of Judaism, gnostic sects, breakaway movements within Islam that teach reincarnation and the manifestation of God in human form. So I wanted to ask him more.

But I couldn’t.

Yazidis like Mirza don’t know even the basic tenets of their faith. These groups’ holy books are carefully hidden from lay people; the priests who read them must promise never to reveal their contents. Entering these leadership ranks isn’t easy, either. Among the Mandaeans of southern Iraq, for example, a people who revere John the Baptist and believe in a colorful array of demons and planetary angels, a would-be priest must spend seven days without food or sleep.

There are at least half a dozen groups in the Middle East which follow this code of secrecy. Their adherents number more than 5 million in Syria, Lebanon, Israel and Iraq. They include Syrian president Bashar al-Assad’s Alawite sect, and also the Yazidis.

Despite this, these groups create a powerful sense of solidarity and identity. Mirza, for example, was an atheist. Yet he still felt strongly tied to his community. When advised by Iraqi politicians that he should convert to Islam if he wanted to protect himself, he told them that he would rather die as a Yazidi.

The tradition of reserving religious secrets to a priestly caste is not new, or peculiar, when viewed against the great sweep of human history.

Plato wrote down much of his philosophy, but kept some of it secret; the Druids in pre-Roman Britain, and the cult of Mithras in the Roman empire, famously refused to divulge their doctrines; and even Jesus held back some of his teaching from the majority of his disciples, explaining his parables fully only to his apostles. The priests of ancient Iran, the Magi, were reputed to have all kinds of secret powers (which is why the word “magic” has come to mean what it does.)

Sometimes the mystery around these religions makes them vulnerable to exploitation. An Alawite runs Syria, but that does not mean good news for his religion. In fact, Bashar al-Assad has done his best to suppress some of the Alawites’ distinctiveness, to bring them into line with the theology of his more orthodox Muslim allies in Tehran.

In fact, keeping these modern-day mystery cults alive in the modern era is going to be hard. They rely on their elders and priests for everything to do with the religion, which can work when they live together in tightly-knit communities; but it means that they often find it very hard to adapt to life in the West.

And the West, in exile, is increasingly where they find themselves. The Yazidis, as I found in August, are desperate to emigrate from Iraq to Europe or anywhere else outside the Middle East. When living in the United States or Europe, however, they can find it hard to hold their communities together. A Yazidi cannot coherently explain or defend his or her religion to classmates or co-workers. Young Druze brought up in the West are often frustrated that they cannot have access to their religion’s rich philosophy.

Yet these faiths are important. They matter more than museum pieces or dying languages because they keep alive ideas and traditions from our remote past. Their hidden philosophies can, when the veil of secrecy is withdrawn, make a contribution to our own debate about the nature of God, to which they have their own startling ideas to contribute. They are also a living disproof of those like the members of the Islamic State, who want to prove that the Middle East and the West have always been at odds. Their disappearance is a tragedy for humanity.