Objection 1. It would seem that Christ was not sanctified in the first instant of His conception. For it is written (1 Corinthians 15:46): "That was not first which is spiritual, but that which is natural: afterwards that which is spiritual." But sanctification by grace is something spiritual. Therefore Christ received the grace of sanctification, not at the very beginning of His conception, but after a space of time.
Objection 2. Further, sanctification seems to be a cleansing from sin: according to 1 Corinthians 6:1: "And such some of you were," namely, sinners, "but you are washed, but you are sanctified." But sin was never in Christ. Therefore it was not becoming that He should be sanctified by grace.
Objection 3. Further, as by the Word of God "all things were made," so from the Word incarnate all men who are made holy receive holiness, according to Hebrews 2:11: "Both he that sanctifieth and they who are sanctified are all of one." But "the Word of God, by whom all things were made, was not Himself made"; as Augustine says (De Trin. i). Therefore Christ, by whom all are made holy, was not Himself made holy.
On the contrary, It is written (Luke 1:35): "The Holy which shall be born of thee shall be called the Son of God"; and (John 10:36): "Whom the Father hath sanctified and sent into the world."
I answer that, As stated above (7, 9,10,12), the abundance of grace sanctifying Christ's soul flows from the very union of the Word, according to John 1:14: "We saw His glory... as it were of the Only-Begotten of the Father, full of grace and truth." For it has been shown above (33, 2,3) that in the first instant of conception, Christ's body was both animated and assumed by the Word of God. Consequently, in the first instant of His conception, Christ had the fulness of grace sanctifying His body and His soul.
Reply to Objection 1. The order set down by the Apostle in this passage refers to those who by advancing attain to the spiritual state. But the mystery of Incarnation is considered as a condescension of the fulness of the Godhead into human nature rather than as the promotion of human nature, already existing, as it were, to the Godhead. Therefore in the man Christ there was perfection of spiritual life from the very beginning.
Reply to Objection 2. To be sanctified is to be made holy. Now something is made not only from its contrary, but also from that which is opposite to it, either by negation or by privation: thus white is made either from black or from not-white. We indeed from being sinners are made holy: so that our sanctification is a cleansing from sin. Whereas Christ, as man, was made holy, because He was not always thus sanctified by grace: yet He was not made holy from being a sinner, because He never sinned; but He was made holy from not-holy as man, not indeed by privation, as though He were at some time a man and not holy; but by negation--that is, when He was not man He had not human sanctity. Therefore at the same time He was made man and a holy man. For this reason the angel said (Luke 1:35): "The Holy which shall be born of thee." Which words Gregory expounds as follows (Moral. xviii): "In order to show the distinction between His holiness and ours, it is declared that He shall be born holy. For we, though we are made holy, yet are not born holy, because by the mere condition of a corruptible nature we are tied... But He alone is truly born holy who... was not conceived by the combining of carnal union."
Reply to Objection 3. The Father creates things through the Son, and the whole Trinity sanctifies men through the Man Christ, but not in the same way. For the Word of God has the same power and operation as God the Father: hence the Father does not work through the Son as an instrument, which is both mover and moved. Whereas the humanity of Christ is as the instrument of the Godhead, as stated above (7, 1, ad 3; 8, 1, ad 1). Therefore Christ's humanity is both sanctified and sanctifier.
Source: Summa Theologica (New Advent)