5 But if He Alone goes to the Father, what does it profit us? Why is the world convinced by the Holy Ghost of this righteousness? And yet if He did not Alone go to the Father, He would not say in another place, “No man has ascended up to heaven, but He That descended from heaven, the Son of man who is in heaven.” But the Apostle Paul also says, “For our conversation is in heaven.” And why is this? Because he also says, “If you be risen with Christ, seek the things which are above, where Christ sits on the right hand of God.
Mind the things which are above, not those which are upon the earth. For you are dead, and your life is hid with Christ in God.” How then is He Alone? Is He therefore Alone because Christ with all His members is One, as the Head with His Body? Now what is His Body, but the Church? As the same teacher says, “Now you are the Body of Christ, and members in particular.” Forasmuch then as we have fallen, and He descended for our sakes, what is, “No man has ascended, but He That descended;” but that no man has ascended, except as made one with Him, and as a member fastened into His Body who descended?
And thus He says to His disciples, “Without Me ye can do nothing.” For in one way is He One with the Father, and in another one with us. He is One with the Father, in that the Substance of the Father and the Son is One; He is One with the Father, in that, “Being in the Form of God, He thought it not robbery to be equal with God.” But He was made One with us, in that “He emptied Himself, taking the form of a servant;” He was made one with us, according to the seed of Abraham, “in whom all nations shall be blessed.”
Which place when the Apostle had brought forward, he said, “He says not, And to seeds, as of many; but as of one, And to your Seed, which is Christ.” And for that we too belong to that which is Christ, by our incorporation together, and coherence to That Head, It is One Christ. And also for that he says to us too, “Therefore are you Abraham's seed, heirs according to the promise.” For if the seed of Abraham be One, and That One Seed of Abraham can only be understood of Christ; but this seed of Abraham we also are; therefore This Whole, that is, the Head and the Body, is One Christ.
Source: Sermons on Selected Lessons of the New Testament (New Advent)