I also said that there were proper Christians around during then that believed in Jesus (pbuh) as Allah's Messenger and thus were monothiests. Even to this day, in the gospel of barnabas the Prophet Muhammad is mentioned by name.
Allah says in Quran he does not punish until the divine message reaches a person, so if we assume this Barak was a polythiest trinitarian and if he knew another Prophet was coming and accepted that new Prophet then he'd rectify his mistakes too according to the new Prophets teachings, so Christians who accept the new Prophet and rectify their mistakes are not considered the blasphemer etc etc as they were just mistaken due to being duped by their priests. Its those who reject the new Prophet that are the blasphemers as they have no excuse to continue what God clarifies in new revelation as blasphemy
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https://youtu.be/-hnNcgXauYw
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And although there's a report that a Christian monk helped confirm that indeed it was Jibril (as) that came to Muhammad (saw), the absolute confirmation was clinched by the Prophets wife Khadija.
Jibril sometimes would appear to Muhammad (saw) in his home and only Muhammad (saw) could see him. So Khadija (ra) told him, "next time he comes tell me'. So when Jibril (as) appeared to Muhammad (saw), he told Khadija, and khadija sat on Muhammad's (saw) lap and uncovered her head and asked Muhammad (saw) if the 'thing' was still there in the room, the Prophet (saw) replied 'no he's left', and Khadija told him then it's not a devil and has to be an angel because an angel would not remain in a room where husband and wife start to get intimate.
She reassured him with other sound advice too, saying, he was a good man, never told lies, gave charity to poor etc etc and Allah wouldn't humiliate him with a curse of devil visits....
So it was his wife that played the 99.9 percent part in confirming his Prophethood.