Objection 1. It would seem that Christ did not acquire His judiciary power by His merits. For judiciary power flows from the royal dignity: according to Proverbs 20:8: "The king that sitteth on the throne of judgment, scattereth away all evil with his look." But it was without merits that Christ acquired royal power, for it is His due as God's Only-begotten Son: thus it is written (Luke 1:32): "The Lord God shall give unto Him the throne of David His father, and He shall reign in the house of Jacob for ever." Therefore Christ did not obtain judiciary power by His merits.
Objection 2. Further, as stated above (Article 2), judiciary power is Christ's due inasmuch as He is our Head. But the grace of headship does not belong to Christ by reason of merit, but follows the personal union of the Divine and human natures: according to John 1:14-16: "We saw His glory... as of the Only-Begotten of the Father, full of grace and truth... and of His fulness we all have received": and this pertains to the notion of headship. Consequently, it seems that Christ did not have judiciary power from merits.
Objection 3. Further, the Apostle says (1 Corinthians 2:15): "The spiritual man judgeth all things." But a man becomes spiritual through grace, which is not from merits; otherwise it is "no more grace," as is said in Romans 11:6. Therefore it seems that judiciary power belongs neither to Christ nor to others from any merits, but from grace alone.
On the contrary, It is written (Job 36:17): "Thy cause hath been judged as that of the wicked, cause and judgment thou shalt recover." And Augustine says (Serm. cxxvii): "The Judge shall sit, who stood before a judge; He shall condemn the truly wicked, who Himself was falsely reputed wicked."
I answer that, There is nothing to hinder one and the same thing from being due to some one from various causes: as the glory of the body in rising was due to Christ not only as befitting His Godhead and His soul's glory, but likewise "from the merit of the lowliness of His Passion" [Cf. Augustine, Tract. civ in Joan.]. And in the same way it must be said that judiciary power belongs to the Man Christ on account of both His Divine personality, and the dignity of His headship, and the fulness of His habitual grace: and yet He obtained it from merit, so that, in accordance with the Divine justice, He should be judge who fought for God's justice, and conquered, and was unjustly condemned. Hence He Himself says (Apocalypse 3:21): "I have overcome and am set down in My Father's throne [Vulgate: 'with My Father in His throne']." Now judiciary power is understood by "throne," according to Psalm 9:5: "Thou hast sat on the throne, who judgest justice."
Reply to Objection 1. This argument holds good of judiciary power according as it is due to Christ by reason of the union with the Word of God.
Reply to Objection 2. This argument is based on the ground of His grace as Head.
Reply to Objection 3. This argument holds good in regard to habitual grace, which perfects Christ's soul. But although judiciary power be Christ's due in these ways, it is not hindered from being His due from merit.
Source: Summa Theologica (New Advent)