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> Sergius was likely Arian Christian.
That doesn't really matter. His meeting with Mohammed is just a passing moment of the Prophet's life. Early life. And he could have left deep impression but not with his teachings, that would have required a master-disciple relationship.
If they even met.
The life of Mohammed and with that the formation of the Islam is "sacred" history which was written in a spirit that emphasizes the divine nature of the events. Sergius' prophecy is a tool in their hands to do exactly this.
It's not surprising that every detail of his life is questionable and unsure. Like that letter.
He had to meet Christians, he most likely heard some teachings here and there. But he lived side by side with the Jews - supposedly - for a decade. He (and his followers who actually created Islam) might got more inspiration from them.