Thus the Exalted One commanded him—after His statement, “Indeed, the religion in Allah’s sight is Islam” (Āl ʿImrān 3:19)—to declare: “I have submitted my face to Allah, and so have those who follow me.” He also instructed him to say to those who had been given the Scripture—that is, the Jews and the Christians—and to the unlettered people, meaning those who possess no revealed book from among the Arabs and others: “Have you submitted?” By unanimous agreement, the unlettered Arabs are included in the expression “the unlettered.”

As for everyone besides them, they are either encompassed by that very term or fall under its meaning through other wording that makes it clear he was sent to all humankind.

The Exalted also said, “If they submit, they are rightly guided; but if they turn away, your duty is only to convey. Allah is All-Seeing of His servants” (Āl ʿImrān 3:20).

Thus He commanded the People of the Book to embrace Islam just as He commanded the unlettered, counting them among the guided if they submit. Yet if they refuse, He said, “Your duty is only to convey,” meaning that you must deliver to them the messages of your Lord, while Allah alone will reckon with them. All of this proves that he had to convey to the People of the Book what Allah required of them regarding Islam just as he conveyed it to the unlettered, and that Allah will hold them accountable for abandoning Islam just as He will hold the unlettered accountable.

Both Ṣaḥīḥ collections record that the Prophet—may Allah bless him and grant him peace—in the letter he sent to Heraclius, the emperor of the Christians, wrote: “From Muḥammad, the Messenger of Allah, to Heraclius…"