The end of this week has special significance for millions of people around the world. Western Christians commemorate Jesus' crucifixion tomorrow and his resurrection on Sunday. Some Muslims celebrate the birth of the prophet Muhammad today and this year the Jewish holiday of Purim begins tonight.

Across Iran, today is Noruz, the day of the vernal equinox, and the start of a new year in the Iranian solar calendar. For Muslim Iranians it doesn't have any religious connotations, though it's a holy day for Zoroastrians. It also forms part of the Baha'i religious calendar, though Baha'is, along with Indian Zoroastrians, celebrate it on Friday.

It's a dizzying array of festivals, rites and observances. But it was the coincidence of Purim, Noruz and Easter falling in the same week that got me thinking about Iran's religious diversity - not a topic you hear a great deal about. Iran is often thought of as a country where only Muslims live - it is, of course, officially the "Islamic Republic". For many, the symbol of the country since the revolution of 1979 is the face of Ayatollah Khomeini, the shia cleric who swept in after the fall of the Shah and set about establishing a form of theocracy.

But Islamic Iran is only part of the story. The Jewish community of Iran is one of the oldest of the diaspora. Its modern representatives claim a direct link with the Jews released from their Babylonian captors by Kurosh - the Achaemenid King Cyrus - in 539 BC. Purim itself commemorates the events described in the book of Esther, the story of the Persian Queen - herself a Jew - who persuaded her husband, Ahasuerus not to carry out his minister Haman's plan to have the Jewish population of the empire murdered.

This is ancient history of course, but Jews have played their part in Iranian society for many centuries and to some extent still do. Outside the country, particularly in the US and Israel, Iranian Jews form a distinct and vocal group. Here's something you might not have realised: the president of Israel between 2000 and 2007 was Iranian-born.

Christianity has a shorter but equally fascinating history in Iran, where it is most strongly associated with the Armenian ethnic minority. In the 17th century, displaced by Iran's wars against the Ottomans, thousands of Armenians settled in Isfahan, where their descendants still live. The magnificent Vank cathedral, which has a mosque-like dome decorated with paintings of cherubs and angels, is a striking and unexpected testament to their faith. Isfahan was also a focus for Iran's small Anglican community for much of the 20th century, led from by 1961 the Iranian Bishop Hassan Dehqani-Tafti. Seen as closely allied to British and American interests, however, it did not survive the revolution. The Bishop, who was shot at and whose son was murdered, left the country soon afterwards and spent the rest of his life in England.

The only faith native to Iran is Zoroastrianism, a monotheistic religion whose founding prophet probably lived during the 10th century BC. The rulers of Iran were all adherents of Zoroaster until the Arab conquest in the 7th century bought Islam in its wake. For non-Zoroastrians, the traditions most closely identified with this group are the worship of fire in temples, and the use of "towers of silence" as a way of disposing of dead bodies. Rather than bury or cremate their dead, Zoroastrians would place them in open-air chambers away from the town, exposed to the sun and to birds of prey - though this is a rite that's no longer practised.

There are about 50,000 Zoroastrians in Iran, and the desert city of Yazd is seen as their heartland. It is for these people that Noruz has a really sacred significance, though Zoroastrian ritual - in form of the Haft Sin - is incorporated into every family's celebration of the event.

So Iran is religiously diverse, something of which this particular "holy week" provides a strong reminder. But it would be wrong to paint too a rosy picture. Contemporary Iran is tolerant of certain religious minorities on paper, but in reality many - like the Jews, whose number have declined from around 100,000 in 1948 to 25,000 in 2004 - prefer to live elsewhere. Ahmadinejad's comments on the holocaust and an official line on Israel which is hugely antagonistic don't help matters.

The post-revolutionary years have likewise seen emigration by Zoroastrians and Christians, who, while they might not be actively harassed, are made to feel excluded by the overwhelmingly Islamic tenor of the state. And to others, of course, the Republic is barbarically intolerant: Baha'ism, a religion founded in the 19th century by the Iranian mystic, Baha'u'llah, is officially regarded as a heresy, and its followers as Muslim apostates. They have been viciously persecuted since 1979, and face arbitrary arrest and even execution.

Ultimately Iran must become a country where freedom of religion is practised without constraint. The path to this state of affairs will be long and incremental, but it is possible simply because the majority of ordinary Iranians believe religion is a private matter.

Until then, with the voluntary and enforced departure of what is left of the faith minorities, the country risks becoming a truly monoreligous state for the first time in its long history.