What Christian’s need to know about Islam: #The Quran was…

What Christian’s need to know about Islam:
#The Quran was not written by Muhammad , he was illiterate. It was orally revealed by God allegedly through an angel. Muhammad memorized it verse by verse before it was put into writing.
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Questions:
1. Who can authentic that the verses Muhammad allegedly memorized were really from God or it was revealed by an angel if there was no one there who can confirm the transmission?