12 But because it is proper to the Father alone and the Son that They should be one by nature because God is from God, and the Only-begotten from the Unbegotten can subsist in no other nature than that of His origin; so that He Who was begotten should exist in the substance of His birth, and the birth should possess no other and different truth of deity than that from which it issued; for our Lord has left us in no doubt as to our belief by asserting throughout the whole of the discourse which follows the nature of this complete unity.
For the next words are these, That the world may believe that You sent Me. Thus the world is to believe that the Son has been sent by the Father because all who shall believe in Him will be one in the Father and the Son. And how they will be so we are soon told— And the glory which You have given Me I have given unto them. Now I ask whether glory is identical with will, since will is an emotion of the mind while glory is an ornament or embellishment of nature.
So then it is the glory received from the Father that the Son has given to all who shall believe in Him, and certainly not will. Had this been given, faith would carry with it no reward, for a necessity of will attached to us would also impose faith upon us. However He has shown what is effected by the bestowal of the glory received, That they may be one, even as We are one. It is then with this object that the received glory was bestowed, that all might be one. So now all are one in glory, because the glory given is none other than that which was received: nor has it been given for any other cause than that all should be one.
And since all are one through the glory given to the Son and by the Son bestowed upon believers, I ask how can the Son be of a different glory from the Father's, since the glory of the Son brings all that believe into the unity of the Father's glory. Now it may be that the utterance of human hope in this case may be somewhat immoderate, yet it will not be contrary to faith; for though to hope for this were presumptuous, yet not to have believed it is sinful, for we have one and the same Author both of our hope and of our faith.
We will treat of this matter more clearly and at greater length in its own place, as is fitting. Yet in the meantime it is easily seen from our present argument that this hope of ours is neither vain nor presumptuous. So then through the glory received and given all are one. I hold the faith and recognise the cause of the unity, but I do not yet understand how it is that the glory given makes all one.
Source: On the Trinity (New Advent)