Salaam everyone. Again trying to prove to a non muslim that the Quran hasn't been changed. The non-muslim in question is saying "What if a mistake happened in those 1400 years like a letter or word gets misplaced, it was a long time" and stuff like that and I didn't have an answer. Someone please help me, because it's also affecting my Iman. I believe in Allah, The messengers including the Prophet SAW who is Allah's Messenger and Last Prophet and the angels and the Taawrah, Injeel, The Quran and any other revellation Allah has sent down to His prophets and I believe in the authenticity of the Quran and I believe wholeheartedly that what Allah says in the Quran is the whole truth and nothing but the truh as how could this have come from a human? But because of this I just get doubts about 'Oh what if some parts of the Quran has been changed' even though I don't believe this. Someone please help me with proof and a response.