Catena Aurea on John 17:14–19: Augustine of Hippo

It is manifest by this, that He is still speaking of the Apostles; for the very word Apostle means in the Greek, sent. But since they are His members, in that He is the Head of the Church, He says, And for their sakes I sanctify Myself; i. e. I in Myself sanctify them, since they are Myself. And to make it more clear that this was His meaning, He adds, That they also might be sanctified through the truth, i. e. in Me; inasmuch as the Word is truth, in which the Son of man was sanctified from the time that the Word was made flesh. For then He sanctified Himself in Himself, i. e. Himself as man, in Himself as the Word: the Word and man being one Christ. But of His members it is that He saith, And for their sakes I sanctify Myself, i. e. them in Me, since in Me both they and I are. That they also might be sanctified in truth: they also, i. e. even as Myself; and in the truth, i. e. Myself.