It’s a heck of a question.

While a small footnote in history, the legitimacy of the letter and indeed the possible existence of the letter itself, opens up some really interesting insights into the mindset of both the early Islamic State and later views of how Islam saw itself and the rest of the world.

Without getting totally lost into the bewildering arguments and counter arguments, we can offer some simplistic renderings of an answer.

Firstly, the Emperor Heraclius is the only Roman Emperor spoken off highly by Muslim sources and one of only a small handful of foreign rulers granted respectful honourifics. Which begs the question why?

Partly this is because Heraclius was around when the Prophet was also active, making him contemporaneous to Mohammed. Secondly, because of the existence of the letter, Heraclius therefore fits the ideal of being someone who Muhammad (and therefore all Muslims) should consider a rival but also a peer; certainly El-Chileikh believed that Muslim writers about him were focused on ‘legitimising [the] role he is made to play... his function as an ideal ‘witness’ to the prophet Muhammad and to the emerging umma’. (P.5)

Within the geopolitical reality of Byzantine-Muslim relations it is worth briefly looking at the context of their supposed relationship. By 628/9 Heraclitus was at the height of his glory. His long war against the Sassanids has ended with the murder and overthrow of Khosrow and he restores the True Cross having regained Syria and Egypt from the Sassanids.

By 641 at his death, most of the Levant and Egypt now lay in the hands of the Muslims under the leadership of the Prophet and the first Amir Abu Bakr (caliph as a title was not used by the leaders of the Muslim until after the 2nd Fit’na but was retrospectively granted to all previous rulers after that).

The question returns then, why was Heraclius granted such honour by the Muslim sources? Well partly we can see a very functional Muslim theological argument for it and the letter supposedly sent out by Muhammad provides the ‘evidence’ for it.

According to tradition the Prophet sent out a bunch of diplomatic letters to a bunch of potentates and leaders calling on them to embrace Islam. When they didn’t the Muslims began their conquests of these lands.

The most spectacular success was against the Sassanids, where internal division, combined with loss of manpower due to war and plague, saw a relatively quick series of victories. Within Muslim ideology, this happened because it was the will of Allah. God has granted them victory because they were fulfilling Gods purpose.

With Rome/Byzantium however, after swift victories in Syria and Egypt, Byzantium managed to resist. Especially after the first Muslim Civil War. By 678 the Muslim siege of Constantinople had failed. It begged the question- what special qualities did Byzantium have that the Sassanids didn’t?

The answer it appeared was the letters of Muhammad.

Muslim tradition dictates that upon receiving his letter, the Sassanids leader tore it up and demanded retribution for the audacity. Heraclius supposedly held the document close to his heart and was moved by it.

This act of respect was later used by Muslim scholars to grant Heraclius Godly agency- THIS was why Byzantium resisted the forces of Islam better than the Sassanids ever did.

And this in turn leads to both the letter and then later Muslim writings about Heraclius to begin to paint him as this idealised enemy. A virtuous man, just and noble, a man fulfilling Allah’s purpose (after all he HAD weakened the Sassanids and the term ‘the enemy of my enemy is my friend’ is an Arab one). According then to those early Muslim writers Heraclius was basically a man who suffered from only one flaw... he refused to convert to Islam (reasons given, greed and his generals wouldn’t allow him).

This leads to wonderfully rich stories of Heraclius basically being a glorious leader of an unjust people; a man who recognised the inherent virtue of Islam; a man who often praised Islam and was convinced they had God on their side.

Given Heraclius known religious sensibilities? We can happily place such things as ‘deep embellishments’ and show how they grant later legitimacy to Muslim scholars in pragmatically explaining away the Faithfuls inability to storm Anatolia (leading to the creation of the vast demarcation zone, the al-Dawāhī (outer lands) or ‘the extremities’ (ta akra) depending if you spoke Arabic or Greek, between the two Empires).

With these embellishments then it becomes very easy to dismiss all the letters sent by Muhammad to be later fabrications, created by Muslim writers to give post-hoc foundation for events that followed and this does seem to be the view held by many.

However...

Not all the letters are treated the same. The letter from the Prophet to the leaders of Yemen is treated as totally legitimate and is a source of some pride for the Yemeni. And all the ‘diplomatic letters’ supposedly sent by Muhammad all have consistencies of style and prose to suggest that they were all written by the same writer.

If one is true, therefore all must be true. This has led to several Muslim scholars insisting upon the authenticity of document with perhaps the most nuanced reply being that of Professor Irfan Sahîd of Georgetown University, who suggests that the letter WAS authentic but that it probably never got to Heraclius and acknowledges that ‘the embroideries surrounding the letter have, of course, to be rejected’ (p531).

Which leaves us with a simple answer of- it’s open to debate, it may well have been written, but there exists no evidence that he ever got it, and the case for its authenticity faces difficulties due to later Muslim elaboration.

Hope that helps.

Sources for this:

Muḥammad and Heraclius: A Study in Legitimacy; Nadia Maria El-Cheikh; Studia Islamica; No89 (1999), pp. 5-21 link here

Review: Arabic Literature to the End of the Umayyad period; Irfan Shahîd; Journal of the American Oriental Society; Vol 106, No. 3, pp. 529-538 link here