Objection 1. It would seem that the soul of Christ was assumed before the flesh by the Word. For the Son of God assumed flesh through the medium of the soul, as was said above (Article 1). Now the medium is reached before the end. Therefore the Son of God assumed the soul before the body.
Objection 2. Further, the soul of Christ is nobler than the angels, according to Psalm 96:8: "Adore Him, all you His angels." But the angels were created in the beginning, as was said above (I, 46, 3). Therefore the soul of Christ also (was created in the beginning). But it was not created before it was assumed, for Damascene says (De Fide Orth. iii, 2,3,9), that "neither the soul nor the body of Christ ever had any hypostasis save the hypostasis of the Word." Therefore it would seem that the soul was assumed before the flesh, which was conceived in the womb of the Virgin.
Objection 3. Further, it is written (John 1:14): "We saw Him [Vulgate: 'His glory'] full of grace and truth," and it is added afterwards that "of His fulness we have all received" (John 1:16), i.e. all the faithful of all time, as Chrysostom expounds it (Hom. xiii in Joan.). Now this could not have been unless the soul of Christ had all fulness of grace and truth before all the saints, who were from the beginning of the world, for the cause is not subsequent to the effect. Hence since the fulness of grace and truth was in the soul of Christ from union with the Word, according to what is written in the same place: "We saw His glory, the glory as it were of the Only-begotten of the Father, full of grace and truth," it would seem in consequence that from the beginning of the world the soul of Christ was assumed by the Word of God.
On the contrary, Damascene says (De Fide Orth. iv, 6): "The intellect was not, as some untruthfully say, united to the true God, and henceforth called Christ, before Incarnation which was of the Virgin."
I answer that, Origen (Peri Archon i, 7,8; ii, 8) maintained that all souls, amongst which he placed Christ's soul, were created in the beginning. But this is not fitting, if we suppose that it was first of all created, but not at once joined to the Word, since it would follow that this soul once had its proper subsistence without the Word; and thus, since it was assumed by the Word, either the union did not take place in the subsistence, or the pre-existing subsistence of the soul was corrupted. So likewise it is not fitting to suppose that this soul was united to the Word from the beginning, and that it afterwards became incarnate in the womb of the Virgin; for thus His soul would not seem to be of the same nature as ours, which are created at the same time that they are infused into bodies. Hence Pope Leo says (Ep. ad Julian. xxxv) that "Christ's flesh was not of a different nature to ours, nor was a different soul infused into it in the beginning than into other men."
Reply to Objection 1. As was said above (Article 1), the soul of Christ is said to be the medium in the union of the flesh with the Word, in the order of nature; but it does not follow from this that it was the medium in the order of time.
Reply to Objection 2. As Pope Leo says in the same Epistle, Christ's soul excels our soul "not by diversity of genus, but by sublimity of power"; for it is of the same genus as our souls, yet excels even the angels in "fulness of grace and truth." But the mode of creation is in harmony with the generic property of the soul; and since it is the form of the body, it is consequently created at the same time that it is infused into and united with the body; which does not happen to angels, since they are substances entirely free from matter.
Reply to Objection 3. Of the fulness of Christ all men receive according to the faith they have in Him; for it is written (Romans 3:22) that "the justice of God is by faith of Jesus Christ unto all and upon all them that believe in Him." Now just as we believe in Him as already born; so the ancients believed in Him as about to be born, since "having the same spirit of faith... we also believe," as it is written (2 Corinthians 4:13). But the faith which is in Christ has the power of justifying by reason of the purpose of the grace of God, according to Romans 4:5: "But to him that worketh not, yet believeth in Him that justifieth the ungodly, his faith is reputed to justice according to the purpose of the grace of God." Hence because this purpose is eternal, there is nothing to hinder some from being justified by the faith of Jesus Christ, even before His soul was full of grace and truth.
Source: Summa Theologica (New Advent)