De Doctrina Christiana Book 1, Chapter X.

Some people call this [Gen 2.15,16,17] “the covenant of works,” though it does not appear from any passage of Scripture to have been either a covenant or of works. Adam was not required to perform any works; he was merely forbidden to do one thing. It was necessary that one thing at least should be forbidden or commanded, and above all something which was in itself neither good nor evil, so that man’s obedience might in this way be made evident. For man was by nature good and holy, and was naturally disposed to do right, so it was certainly not necessary to bind him by the requirements of any covenant to something which he would do of his own accord. And he would not have shown obedience at all by performing good works, since he was in fact drawn to these by his own natural impulses, without being commanded. Besides a command, whether it comes from God or from a magistrate, should not be called a covenant just because rewards and punishments are attached: it is rather a declaration of power.

It was called the tree of knowledge of good and evil because of what happened afterwards: for since it was tasted, not only do we know evil, but also we do not even know good except through evil.