Man is the measure of all things: of things which are, that they are; and of things which are not, that they are not.
Where Protagoras messed up wasn't in implying the ability to chose which is placed in us (i.e. free will) but rather the degree to which he elevated that free will. Solomon wrote it very clearly when he said, "Fear God and keep His commandments, for this is man's all."
While we do have the ability to decide what we will do, we do not have the ability to chose whether that thing is right or wrong - it has already been decided.
That being said, let's think about that in light of today. It's April 1, 2014 which means today is the day of the practical joker, and I plan on taking my holiday to heart and celebrating it with passion. That being said, there was once another very wise, inspired man who wrote another passage of Scripture stating that the fool, or the wicked man, has determined to reject the overwhelming evidence of his Creator and Lord and deem his own free will as the measure of all things.
It's extremely sad to see our world in its current situation. God is mocked as if He were the intended victim of April Fool's Day. He is cursed and condemned even though He is the condemner. And yet all the while He's looking down on us pleading with our race to reject the words of a man (Protagoras or any other relativistic philosopher) and receive the words of the Master.
While you are playing jokes today, don't forget that your acceptance of God and obedience to Him is no joke!