How the Koran guides man?

The Koran itself confirms that all inside is said by God and not the prophet (Muhammad) he is only the messenger and expresses whatever is revealed to him by the angel and God.
The Koran is a Divine Book reveled to guide man towards the Truth:

1. We have sent down a Book to you in order bring men out of darkness into Light by their Lords permission, towards the road of the powerful the praiseworthy! (14:1)
2. They do not value God the way He should be valued when they say: “God has not sent down anything to a human being. “Say” who sent down the Book which Moses came with, as Light and guidance for mankind? You   publish it on (separate) sheets to show it around. While you (also) hide a great deal [of it] you have been taught what neither you yourselves nor forefathers ever know. “Say” God [has sent it] then leave them toying away with their speculation.(6:91)
3. We send down something from the Koran [to serve] as healing and a mercy for believers, while wrongdoers are only increased in loss.(17:82)