Theophylact of Ohrid — as excerpted in the Catena Aurea on John 8:52-56

“Having that knowledge by nature; for as I am, so is the Father also; I know Myself, and therefore I know Him. And He gives the proof that He knows Him: And I keep His saying, i. e. His commandments. Some understand, I keep His saying, to mean, I keep the nature of His substance unchanged; for the substance of the Father and the Son is the same, as their nature is the same; and therefore I know the Father. And here has the force of because: I know Him because I keep His saying.”