Philosophers like Nietzsche (''Thus Spake Zarathustra'') have seen the prophet, with his emphasis on reason and morality, as a precursor of Plato. Religious historians say Zoroastrianism had a broad influence on Judeo-Christian thought, contributing to such concepts as individual responsibility for one's acts, divine justice, heaven and hell, Satan, a Saviour, Last Judgment, Resurrection and life everlasting for the reunited soul and body, as well as monotheism.

At the time of Jesus, Zoroastrianism, as the state religion of the Persian Empire, was probably the dominant faith in the Middle East. Cyrus and Xerxes are thought to have practiced a form of it. Then, in the mid-seventh century, Moslem Arabs conquered Persia. There were persecutions and conversions by force. Several thousand Zoroastrians remained behind in what is now Iran, but another part of the community relocated to the Indian subcontinent. This Indian branch, eventually the majority of Zoroastrians, became known as the Parsis -i.e., from Persia.

Parsis who flocked to Bombay traded and acted as mid-dlemen between the British and the native population. The community flourished: some became millionaires and philanthropists; some were knighted. The golden age of Parsi entrepreneurship may have passed with the Raj. Or the enterprising spirit may have shifted overseas, principally to North America. This time, the diaspora is fueled by the lure of advanced technology and economic and artistic opportunity. Zubin Mehta, the conductor, and Persis Khambatta, the actress, are among the well-known Parsis living in America today. It is this new American community - talented, confident, educated, middle-class to affluent -that has begun to question the authority of the religious councils back home.

W HEN WE LAST-ed in countries where there was persecution, why can't we survive in a free country?'' Kersey Antia, a psychologist, asked me rhetorically. ''Because people panic from excess of choice.'' Perhaps, also, the very ease with which the Zoroastrians have adapted to American life -their mastery of the cultural and economic codes - en-dangers their spiritual identity. Before, in Iran or India, they had an insular community where each generation could absorb the religion by osmosis. Over here, isolated in suburban homes, Zoroastrians are facing the same dilemma other ethnic groups have encountered here: how not to dissolve in the solvent of American mass culture.

''Many of the things we believe in seem, in this country, to be shabby,'' says a professor of mechanical engineering at the University of Houston who also teaches at a Zoroastrian Sunday school. ''Take the kushti and sudre: it is unpopular with our children because when they go into gym class they get embarrassed. But the Christian children wear a cross, so why should our children be excused?

''I was sad to see some women not covering their heads or wearing the sari at a navjote. These are aids in practicing the religion. God's not going to strike you dead in any case. But what's to prevent us, instead of having a fire at services, from using a light bulb, or a videotape of a fire, because it's easier to get around the fire codes? Next we can throw out the priests and play a tape recording!''

But isn't the sari, I asked, an accouterment picked up from the Parsis' adaptation to Indian culture and not an essential part of Zoroastrianism?