With possible roots dating back to the Second Millennium BCE, Zoroastrianism enters written history in the 5th century BCE. It served as the state religion of the ancient Iranian empires for more than a millennium, from around 600 BCE to 650 CE, but declined from the 7th century CE onwards following the Muslim conquest of Persia of 633–654 and subsequent persecution of the Zoroastrian people. Recent estimates place the current number of Zoroastrians at around 110,000–120,000 at most, with the majority living in India, Iran, and North America; their number has been thought to be declining.