This perhaps one of Kierkegaard’s more famous quotes and well-known for good reason. Here we see with pointed clarity the difference between knowledge and understanding; between objective and subjective. It is taken from Three Discourses on Imagined Occasions. Note that Kierkegaard does not assume that knowledge deceives but that that being deceived by much knowledge is dreadful.
If you actually are further along, then do not let yourself be delayed, but if not, then consider that one is dreadfully deceived if one is deceived by much knowledge. Let us imagine a firstmate and assume that he has passed with distinction all the examinations but as yet has not been out to sea. Imagine him in a storm: he knows exactly what he has to do, but he is unacquainted with the terror that grips the sailor when the stars disappear into the pitch darkness of the night; he is unacquainted with the sense of powerlessness the pilot feels when he sees that the helm in his hand is only a plaything for the sea; he does not know how the blood rushes to the head when in such a moment one must make calculations – in short, he has no conception of the change that takes place in the knower when he is to use his knowledge (36).