Augustus
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Its been noted that he was quite well traveled and had plenty of interaction with the Jews and Christians of the day. Its not hard to wonder if maybe although he may have not been able to read, took advantage of his interactions with people, to learn their theologies and philosophies.
Also worth noting that the initial audience for the Quran, Muhammad's peers were also clearly familiar with Syriac Christian theologies and narratives. The Quran refers to these frequently, yet never explains them. You don't make reference to things that your audience don't understand without explanation, so it is pretty safe to assume there was no need to explain as they were intimately familiar.
I think it is more revealing to look at what the audience is assumed to know rather than looking at what Muhammad could have learned.
Arabia was to a significant extent monotheist before Muhammad, and many Arabs had a high degree of interaction with the Romans and Persians who had been paying them as mercenaries for a couple of centuries. The 'isolated pagan backwater' idea is mythical, although pagan communities might well have existed alongside monotheistic ones.