The Lord Jesus does, indeed, say to His disciples, as was read in the Gospel lection, if you loved Me, you would assuredly rejoice, because I go to the Father, because the Father is greater than I.
But those ears, which have often heard the words, I and the Father are One, and He that sees Me, sees the Father also, accept the saying without supposing a difference of Godhead or understanding it of that Essence which they know to be co-eternal and of the same nature with the Father.
Man’s uplifting, therefore, in the Incarnation of the Word, is commended to the holy Apostles also.
And they, who were distressed at the announcement of the Lord’s departure from them, are incited to eternal joy over the increase in their dignity; If you loved Me, He says, you would assuredly rejoice, because I go to the Father.
That is, if, with complete knowledge you saw what glory is bestowed on you by the fact that, being begotten of God the Father, I have been born of a human mother also, that being invisible I have made Myself visible, that being eternal in the form of God I accepted the form of a slave, you would rejoice because I go to the Father.
For to you is offered this ascension, and your humility is in Me raised to a place above all heavens at the Father’s right hand.
But I, Who am with the Father that which the Father is, abide undivided with My Father, and in coming from Him to you I do not leave Him, even as in returning to Him from you I do not forsake you.
Rejoice, therefore, because I go to the Father, because the Father is greater than I. For I have united you with Myself, and have become Son of Man that you might have power to be sons of God.
And hence, though I am One in both forms, yet in that whereby I am conformed to you I am less than the Father, whereas in that whereby I am not divided from the Father I am greater even than Myself.
And so let the Nature, which is less than the Father, go to the Father, that the Flesh may be where the Word always is, and that the one faith of the Catholic Church may believe that He Whom as Man it does not deny to be less, is equal as God with the Father.
Leo the Great (c.400-461): Sermon 77, 5.