How is holy quran considered a mercy to all humanity?

There is “Wisdom” & “Guidance” in the Holy Qura’an for every sensible being; who can understand and think, to make use of them. That is why it is called Mercy for the whole Humanity and not just for Muslims.

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Because it is a book that has the keys to success. It was sent down for guidance for everyone on this Earth. As it also states in the Quran itself in Chapter 81 Verse 27. "Verily this is no less than a message to (all the worlds)."  If you read the Quran you would understand and learn new things and become a better person InshAllah.