54 But if His equality is doubted because the Name is given Him after He put off the form of God, we dishonour Him by ignoring the mystery of the humility which He assumed. The birth of His humanity brought to Him a new nature, and His form was changed in His humility, by the assumption of a servant's form, but now the giving of the Name restores to Him equality of form. Ask yourself what it is, which is given. If the gift be something pertaining to God, the grant to the receiving nature does not impair the divinity of the giving nature.
Again, the words, And gave Him the Name, involve a mystery in the giving, but the giving of the Name does not make it another name. To Jesus is given, that to Him, Every knee shall bow of things in heaven, and things on earth, and things under the earth, and every tongue confess that Jesus is Lord in the glory of God the Father. The honour is given Him that He should be confessed in the glory of God the Father. Do you hear Him say, The Father is greater than I? Know Him also, of Whom it is said in reward of His obedience, And gave unto Him the Name which is above every name; hear Him Who said, I and the Father are one; He that has seen Me, has seen the Father also; I am in the Father, and the Father in Me.
Consider the honour of the confession which is granted Him, that Jesus is Lord in the glory of God the Father. When, then, is the Father greater than the Son? Surely, when He gives Him the Name above every name. And on the other hand, when is it that the Son and the Father are one? Surely, when every tongue confesses that Jesus is Lord in the glory of God the Father. If, then, the Father is greater through His authority to give, is the Son less through the confession of receiving?
The Giver is greater: but the Receiver is not less, for to Him it is given to be one with the Giver. If it is not given to Jesus to be confessed in the glory of God the Father, He is less than the Father. But if it is given Him to be in that glory, in which the Father is, we see in the prerogative of giving, that the Giver is greater, and in the confession of the gift, that the Two are One. The Father is, therefore, greater than the Son: for manifestly He is greater, Who makes another to be all that He Himself is, Who imparts to the Son by the mystery of the birth the image of His own unbegotten nature, Who begets Him from Himself into His own form, and restores Him again from the form of a servant to the form of God, Whose work it is that Christ, born God according to the Spirit in the glory of the Father, but now Jesus Christ dead in the flesh, should be once more God in the glory of the Father.
When, therefore, Christ says that He is going to the Father, He reveals the reason why they should rejoice if they loved Him, because the Father is greater than He.
Source: On the Trinity (New Advent)