Not Lawful to Go Beyond the Simplicity of the Gospel

From John Calvin's Institutes (2.15.2):

Outside Christ there is nothing worth knowing, and all who by faith perceive what he is like have grasped the whole immensity of heavenly benefits. For this reason, Paul writes in another passage: "I decided to know nothing precious . . . except Jesus Christ and him crucified" (I Cor. 2:2 ). This is very true, because it is not lawful to go beyond the simplicity of the gospel. And the prophetic dignity in Christ leads us to know that in the sum of doctrine as he has given it to us all parts of perfect wisdom are contained.