8 But perhaps we shall have some fear in saying that He was predestinated, because the apostle seems to have said so only in reference to our being made conformable to His image. As if, indeed, any one, faithfully considering the rule of faith, were to deny that the Son of God was predestinated, who yet cannot deny that He was man. For it is rightly said that He was not predestinated in respect of His being the Word of God, God with God. For how could He be predestinated, seeing He already was what He was, without beginning and without ending, everlasting?
But that, which as yet was not, had to be predestinated, in order that it might come to pass in its time, even as it was predestinated so to come before all times. Accordingly, whoever denies predestination of the Son of God, denies that He was also Himself the Son of man. But, on account of those who are disputatious, let us also on this subject listen to the apostle in the exordium of his epistles. For both in the first of his epistles, which is that to the Romans, and in the beginning of the epistle itself, we read: “Paul, a servant of Jesus Christ, called [to be] an apostle, separated unto the gospel of God, which He had promised afore by His prophets in the Holy Scriptures, concerning His Son, who was made for Him of the seed of David according to the flesh, who was predestinated the Son of God in power, according to the Spirit of holiness, by the resurrection from the dead.” In respect, then, of this predestination also, He was gloried before the world was, in order that His glory might be, by the resurrection from the dead, with the Father, at whose right hand He sits.
Accordingly, when He saw that the time of this, His predestinated glorification, was now come, in order that what had already been done in predestination might also be done now in actual accomplishment, He said in His prayer, “And now, O Father, glorify Thou me with Your own self with the glory which I had with You before the world was:” as if He had said, The glory which I had with You, that is, that glory which I had with You in Your predestination, it is time that I should have with You also in sitting at Your right hand. But as the discussion of this question has already kept us long, what follows must be taken into consideration in another discourse.
Source: Tractates on the Gospel of John (New Advent)