49 Connect with this that saying, which you lay hold of to support the imputation of infirmity, All that the Father gives Me shall come unto Me, and him that comes to Me I will in no wise cast out; for I have come down from heaven not to do My own will, but the will of the Father that sent Me. But, perhaps you say, the Son has no freedom of will: the weakness of His nature subjects Him to necessity, and He is denied free-will, and subjected to necessity that He may not reject those who are given to Him and come from the Father.
Nor was the Lord content to demonstrate the mystery of the Unity by His action in not rejecting those who are given to Him, nor seeking to do His own will instead of the will of him that sent Him, but when the Jews, after the repetition of the words, Him that sent Me, began to murmur, He confirms our interpretation by saying, Every one who hears from the Father and learns, comes unto Me. Not that any man has seen the Father, save He which is from God, He has seen the Father.
Verily, verily, I say unto you, he that believes in Me has eternal life</em>. Now, tell me first, where has the Father been heard, and where has He taught His hearers? No one has seen the Father, save Him Who is from God: has any one ever heard Him Whom no one has ever seen? He that has heard from the Father, comes to the Son: and he that has heard the teaching of the Son, has heard the teaching of the Father's nature, for its properties are revealed in the Son. When, therefore, we hear the Son teaching, we must understand that we are hearing the teaching of the Father.
No one has seen the Father, yet he who comes to the Son, hears and learns from the Father to come: it is manifest, therefore, that the Father teaches through the words of the Son, and, though seen of none, speaks to us in the manifestation of the Son, because the Son, by virtue of His perfect birth, possesses all the properties of His Father's nature. The Only-begotten God desiring, therefore, to testify of the Father's authority, yet inculcating His own unity with the Father's nature, does not cast out those who are given to Him of the Father, or work His own will instead of the will of Him that sent Him: not that He does not will what He does, or is not Himself heard when He teaches; but in order that He may reveal Him Who sent Him, and Himself the Sent, under the aspect of one indistinguishable nature, He shows all that He wills, and says, and does, to be the will and works of the Father.
Source: On the Trinity (New Advent)