Objection 1. It would seem that Christ is not subject to Himself. For Cyril says in a synodal letter which the Council of Ephesus (Part I, ch. xxvi) received: "Christ is neither servant nor master of Himself. It is foolish, or rather impious, to think or say this." And Damascene says the same (De Fide Orth. iii, 21): "The one Being, Christ, cannot be the servant or master of Himself." Now Christ is said to be the servant of the Father inasmuch as He is subject to Him. Hence Christ is not subject to Himself.
Objection 2. Further, servant has reference to master. Now nothing has a relation to itself, hence Hilary says (De Trin. vii) that nothing is like or equal to itself. Hence Christ cannot be said to be the servant of Himself, and consequently to be subject to Himself.
Objection 3. Further, "as the rational soul and flesh are one man; so God and man are one Christ," as Athanasius says (Symb. Fid.). Now man is not said to be subject to himself or servant to himself or greater than himself because his body is subject to his soul. Therefore, Christ is not said to be subject to Himself because His Manhood is subject to His Godhead.
On the contrary, Augustine says (De Trin. i, 7): "Truth shows in this way" (i.e. whereby the Father is greater than Christ in human nature) "that the Son is less than Himself."
Further, as he argues (De Trin. i, 7), the form of a servant was so taken by the Son of God that the form of God was not lost. But because of the form of God, which is common to the Father and the Son, the Father is greater than the Son in human nature. Therefore the Son is greater than Himself in human nature.
Further, Christ in His human nature is the servant of God the Father, according to John 20:17: "I ascend to My Father and to your Father to My God and your God." Now whoever is the servant of the Father is the servant of the Son; otherwise not everything that belongs to the Father would belong to the Son. Therefore Christ is His own servant and is subject to Himself.
I answer that, As was said above (1, ad 2), to be master or servant is attributed to a person or hypostasis according to a nature. Hence when it is said that Christ is the master or servant of Himself, or that the Word of God is the Master of the Man Christ, this may be understood in two ways. First, so that this is understood to be said by reason of another hypostasis or person, as if there was the person of the Word of God ruling and the person of the man serving; and this is the heresy of Nestorius. Hence in the condemnation of Nestorius it is said in the Council of Ephesus (Part III, ch. i, anath. 6): "If anyone say that the Word begotten of God the Father is the God or Lord of Christ, and does not rather confess the same to be at once God and man as the Word made flesh, according to the Scriptures, let him be anathema." And in this sense it is denied by Cyril and Damascene (Objection 1); and in the same sense must it be denied that Christ is less than Himself or subject to Himself. Secondly, it may be understood of the diversity of natures in the one person or hypostasis. And thus we may say that in one of them, in which He agrees with the Father, He presides and rules together with the Father; and in the other nature, in which He agrees with us, He is subject and serves, and in this sense Augustine says that "the Son is less than Himself."
Yet it must be borne in mind that since this name "Christ" is the name of a Person, even as the name "Son," those things can be predicated essentially and absolutely of Christ which belong to Him by reason of the Person, Which is eternal; and especially those relations which seem more properly to pertain to the Person or the hypostasis. But whatever pertains to Him in His human nature is rather to be attributed to Him with a qualification; so that we say that Christ is simply greatest, Lord, Ruler, whereas to be subject or servant or less is to be attributed to Him with the qualification, in His human nature.
Reply to Objection 1. Cyril and Damascene deny that Christ is the head of Himself inasmuch as this implies a plurality of supposita, which is required in order that anyone may be the master of another.
Reply to Objection 2. Simply speaking it is necessary that the master and the servant should be distinct; yet a certain notion of mastership and subservience may be preserved inasmuch as the same one is master of Himself in different respects.
Reply to Objection 3. On account of the divers parts of man, one of which is superior and the other inferior, the Philosopher says (Ethic. v, 11) that there is justice between a man and himself inasmuch as the irascible and concupiscible powers obey reason. Hence this way a man may be said to be subject and subservient to Himself as regards His different parts.
To the other arguments, the reply is clear from what has been said. For Augustine asserts that the Son is less than, or subject to, Himself in His human nature, and not by a diversity of supposita.
Source: Summa Theologica (New Advent)