Objection 1. It would seem that Christ will not judge under the form of His humanity. For judgment requires authority in the judge. Now Christ has authority over the quick and the dead as God, for thus is He the Lord and Creator of all. Therefore He will judge under the form of His Godhead.
Objection 2. Further, invincible power is requisite in a judge; wherefore it is written (Ecclesiastes 7:6): "Seek not to be made a judge, unless thou have strength enough to extirpate iniquities." Now invincible power belongs to Christ as God. Therefore He will judge under the form of the Godhead.
Objection 3. Further, it is written (John 5:22-23): "The Father... hath given all judgment to the Son, that all men may honor the Son as they honor the Father." Now equal honor to that of the Father is not due to the Son in respect of His human nature. Therefore He will not judge under His human form.
Objection 4. Further, it is written (Daniel 7:9): "I beheld till thrones were placed and the Ancient of days sat." Now the thrones signify judicial power, and God is called the Ancient by reason of His eternity, according to Dionysius (Div. Nom. x). Therefore it becomes the Son to judge as being eternal; and consequently not as man.
Objection 5. Further, Augustine says (Tract. xix in Joan.) that "the resurrection of the soul is the work of the Word the Son of God, and the resurrection of the body is the work of the Word made the Son of man in the flesh." Now that last judgment regards the soul rather than the body. Therefore it becomes Christ to judge as God rather than as man.
On the contrary, It is written (John 5:27): "He hath given Him power to do judgment, because He is the Son of man."
Further, it is written (Job 36:17): "Thy cause hath been judged as that of the wicked--by Pilate" according to a gloss--therefore, "cause and judgment thou shalt recover--that thou mayest judge justly," according to the gloss. Now Christ was judged by Pilate with regard to His human nature. Therefore He will judge under the human nature.
Further, to Him it belongs to judge who made the law. Now Christ gave us the law of the Gospel while appearing in the human nature. Therefore He will judge under that same nature.
I answer that, Judgment requires a certain authority in the judge. Wherefore it is written (Romans 14:4): "Who art thou that judgest another man's servant?" Hence it is becoming that Christ should judge in respect of His having authority over men to whom chiefly the last judgment will be directed. Now He is our Lord, not only by reason of the Creation, since "the Lord He is God, He made us and not we ourselves" (Psalm 99:3), but also by reason of the Redemption, which pertains to Him in respect of His human nature. Wherefore "to this end Christ died and rose again, that He might be Lord both of the dead and of the living" (Romans 14:9). But the goods of the Creation would not suffice us to obtain the reward of eternal life, without the addition of the boon of the Redemption, on account of the obstacle accruing to created nature through the sin of our first parent. Hence, since the last judgment is directed to the admission of some to the kingdom, and the exclusion of others therefrom, it is becoming that Christ should preside at that judgment under the form of His human nature, since it is by favor of that same nature's Redemption that man is admitted to the kingdom. In this sense it is stated (Acts 10:42) that "He... was appointed by God to be Judge of the living and of the dead." And forasmuch as by redeeming mankind He restored not only man but all creatures without exception--inasmuch as all creatures are bettered through man's restoration, according to Colossians 1:20, "Making peace through the blood of His cross, both as to things on earth, and the things that are in heaven"--it follows that through His Passion Christ merited lordship and judicial power not over man alone, but over all creatures, according to Matthew 28:18, "All power is given to Me, in heaven and in earth" [Cf. III, 59].
Reply to Objection 1. Christ, in respect of His Divine nature, has authority of lordship over all creatures by right of creation; but in respect of His human nature He has authority of lordship merited through His Passion. The latter is secondary so to speak and acquired, while the former is natural and eternal.
Reply to Objection 2. Although Christ as man has not of Himself invincible power resulting from the natural power of the human species, nevertheless there is also in His human nature an invincible power derived from His Godhead, whereby all things are subjected under His feet (1 Corinthians 15:25-28; Hebrews 2:8-9). Hence He will judge in His human nature indeed, but by the power of His Godhead.
Reply to Objection 3. Christ would not have sufficed for the redemption of mankind, had He been a mere man. Wherefore from the very fact that He was able as man to redeem mankind, and thereby obtained judicial power, it is evident that He is God, and consequently is to be honored equally with the Father, not as man but as God.
Reply to Objection 4. In that vision of Daniel the whole order of the judicial power is clearly expressed. This power is in God Himself as its first origin, and more especially in the Father Who is the fount of the entire Godhead; wherefore it is stated in the first place that the "Ancient of days sat." But the judicial power was transmitted from the Father to the Son, not only from eternity in respect of the Divine nature, but also in time in respect of the human nature wherein He merited it. Hence in the aforesaid vision it is further stated (Daniel 7:13-14): "Lo, one like the Son of man came with the clouds of heaven, and He came even to the Ancient of days... And He gave Him power and glory, and a kingdom."
Reply to Objection 5. Augustine is speaking by a kind of appropriation, so as to trace the effects which Christ wrought in the human nature to causes somewhat similar to them. And since we are made to the image and likeness of God in respect of our soul, and are of the same species as the man Christ in respect of our body, he ascribes to the Godhead the effects wrought by Christ in our souls, and those which He wrought or will work in our bodies he ascribes to His flesh; although His flesh, as being the instrument of His Godhead, has also its effect on our souls as Damascene asserts (De Fide Orth. iii, 15), according to the saying of Hebrews 9:14, that His "blood" hath cleansed "our conscience from dead works." And thus that "the Word was made flesh" is the cause of the resurrection of souls; wherefore also according to His human nature He is becomingly the Judge not only of bodily but also of spiritual goods [Cf. III, 56, 2, ad 1].
Source: Summa Theologica (New Advent)