“In the name of Allah, the Beneficent, the Merciful.

From Muhammad the servant of Allah and His Messenger to Heraclius, the great ruler of the Romans.

Peace be upon the one who follows Guidance.

I am calling you with the call of Islam.
Be a Muslim and you will be safe;
Be a Muslim and Allah will double your reward;
But if you turn away, you shall bear the sins of the Arisiyyin.
“O People of the Scriptures! Come to a word common to you and us that we worship none but Allah and that we associate nothing in worship with Him, and that none of us shall take others as Lords beside Allah. Then if they turn away, say: Bear witness that we are Muslims.”

The letter was stamped; first with the word “Allah”, then below it “Messenger” and below that “Muhammad”. The stamp stands for “Muhammad Messenger of Allah”
It feels to me like stating ‘Three’, but not ‘Trinity’. Or separating the ‘Three’, but futher and more clearly that Arius (born ~256CE– died ~336CE) managed to do.
Most of the early Muslim scholars understood the word “Arisiyyin” to mean common people or farmers. However, there are those few like Ibn Hazm in Andalusia and aTahawi in Egypt, who understood it to mean the followers of Arius. Maybe, it even means both.
Whatever is the meaning, it is clear that a ruler has a duty of care towards the common people.
Heraclius, who received this letter, was an Eastern Roman Emperor and there are sound evidence in the Islamic literature that refers to it. So do not be surprised if there is a lack of references to it in the Latin world. For in Jesus of Nazareth,peace be upon him, you have a great example. He is scarcely referenced in the Jewish tradition. So much so that some people doubt even his existance.