Every man, ever born in the flesh, is in a certain sense from God. How then could He say that they were ignorant who He was, and whence He wasa? Because our Lord is here referring to His own peculiar birth from God, which they were ignorant of, because they did not know that He was the Son of God. His very saying then that they did not know whence He was, was telling them whence He was. If they did not know whence He was, He could not be from nothing; for then there would be no whence to be ignorant of. He must therefore be from God. And then not knowing whence He is, was the reason that they did not know who He is. He does not know the Son who does not know His birth from the Father.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Trin. ult. med.