Objection 1. It would seem that Christ's Passion is to be attributed to His Godhead; for it is written (1 Corinthians 2:8): "If they had known it, they would never have crucified the Lord of glory." But Christ is the Lord of glory in respect of His Godhead. Therefore Christ's Passion is attributed to Him in respect of His Godhead.
Objection 2. Further, the principle of men's salvation is the Godhead Itself, according to Psalm 36:39: "But the salvation of the just is from the Lord." Consequently, if Christ's Passion did not appertain to His Godhead, it would seem that it could not produce fruit in us.
Objection 3. Further, the Jews were punished for slaying Christ as for murdering God Himself; as is proved by the gravity of the punishment. Now this would not be so if the Passion were not attributed to the Godhead. Therefore Christ's Passion should be so attributed.
On the contrary, Athanasius says (Ep. ad Epict.): "The Word is impassible whose Nature is Divine." But what is impassible cannot suffer. Consequently, Christ's Passion did not concern His Godhead.
I answer that, As stated above (2, 1,2,3,6), the union of the human nature with the Divine was effected in the Person, in the hypostasis, in the suppositum, yet observing the distinction of natures; so that it is the same Person and hypostasis of the Divine and human natures, while each nature retains that which is proper to it. And therefore, as stated above (Question 16, Article 4), the Passion is to be attributed to the suppositum of the Divine Nature, not because of the Divine Nature, which is impassible, but by reason of the human nature. Hence, in a Synodal Epistle of Cyril [Act. Conc. Ephes., P. i, cap. 26] we read: "If any man does not confess that the Word of God suffered in the flesh and was crucified in the flesh, let him be anathema." Therefore Christ's Passion belongs to the "suppositum" of the Divine Nature by reason of the passible nature assumed, but not on account of the impassible Divine Nature.
Reply to Objection 1. The Lord of glory is said to be crucified, not as the Lord of glory, but as a man capable of suffering.
Reply to Objection 2. As is said in a sermon of the Council of Ephesus [P. iii, cap. 10, "Christ's death being, as it were, God's death"--namely, by union in Person--"destroyed death"; since He who suffered "was both God and man. For God's Nature was not wounded, nor did It undergo any change by those sufferings."
Reply to Objection 3. As the passage quoted goes on to say: "The Jews did not crucify one who was simply a man; they inflicted their presumptions upon God. For suppose a prince to speak by word of mouth, and that his words are committed to writing on a parchment and sent out to the cities, and that some rebel tears up the document, he will be led forth to endure the death sentence, not for merely tearing up a document, but as destroying the imperial message. Let not the Jew, then, stand in security, as crucifying a mere man; since what he saw was as the parchment, but what was hidden under it was the imperial Word, the Son by nature, not the mere utterance of a tongue."
Source: Summa Theologica (New Advent)