By the People of the Book is meant all those who profess a divine religion and consider themselves the followers of one of the prophets of Allah.
Question 315: What are the religions whose followers are considered the People of the Book?
Answer: By the People of the Book is meant all those who profess a divine religion and consider themselves the followers of one of the prophets of Allah, the Glorious and the Exalted (may peace be upon our Prophet and his progeny and upon them) and who possess a heavenly scripture from those revealed to the Prophets (a.), such as the Jews, the Christians, the Zoroastrians and similarly the Sabeans who, on the basis of our research, are among the People of the Book.[1] the followers of the past revealed religions (that is, the Jews, the Christians and the Zoroastrians).[2]
1. Ayatollah Seyyed Ali Khamenei, Practical Laws of Islam, Rules of Non-Muslims, Q315.
2. Ayatollah Seyyed Ali Sistani, A Code of Practice For Muslims in the West, General Rules.