Religious beliefs in the Eastern Empire and Persia were in flux during the late 6th and early 7th centuries. Judaism was an active proselytising faith, and at least one Arab political leader converted to it.[J] Christianity had active missions competing with the Persians' Zoroastrianism in seeking converts, especially among residents of the Arabian Peninsula. All these strands came together with the emergence of Islam in Arabia during the lifetime of Muhammad (d. 632). After his death, Islamic forces conquered much of the Eastern Empire and Persia, starting with Syria in 634–635 and reaching Egypt in 640–641, Persia between 637 and 642, North Africa in the later 7th century, and the Iberian Peninsula in 711. By 714, Islamic forces controlled much of the peninsula in a region they called Al-Andalus.
What was the dominant religion of Persia before Islam?
Zoroastrianism