the path of revelation is protected against mistakes

The path of revelation is part of the Creator's programme. He never makes mistakes, neither in His Creation nor in the system of belief and the laws of the shari'ah, which are delineated for man through revelation.
god says in lxxii:26-28, (He is) the knower of the unseen and he reveals His secret to no one exccpt to every messenger He has chosen and He makes a guard go before him and a guard behind him, that He may know that they have indeed conveyed the message of the Lord. He surrounds all their doings and he keeps count of all things.
From this we understand that the prophets and messengers of God must be infallible both in receiving the revelation and in preserving it against alteration and attack. They are as instruments at the disposal of the Creator's wisdom. Were they to make an error in receiving or teaching the message of the revelation or be led astray by the whispering of evil persons, were they themselves to commit wrong or deliberately change the message they had to deliver, then the wisdom of God would be unable to perfect its programme of guidance.
God confirms in chapter XVI:9 that He is in total control of man's guidance by means of his messenger, "And God's is the direction of the way, and some (words) do not go straight."