Objection 1. It would seem that there was ignorance in Christ. For that is truly in Christ which belongs to Him in His human nature, although it does not belong to Him in His Divine Nature, as suffering and death. But ignorance belongs to Christ in His human nature; for Damascene says (De Fide Orth. iii, 21) that "He assumed an ignorant and enslaved nature." Therefore ignorance was truly in Christ.
Objection 2. Further, one is said to be ignorant through defect of knowledge. Now some kind of knowledge was wanting to Christ, for the Apostle says (2 Corinthians 5:21) "Him that knew no sin, for us He hath made sin." Therefore there was ignorance in Christ.
Objection 3. Further, it is written (Isaiah 8:4): "For before the child know to call his Father and his mother, the strength of Damascus... shall be taken away." Therefore in Christ there was ignorance of certain things.
On the contrary, Ignorance is not taken away by ignorance. But Christ came to take away our ignorance; for "He came to enlighten them that sit in darkness and in the shadow of death" (Luke 1:79). Therefore there was no ignorance in Christ.
I answer that, As there was the fulness of grace and virtue in Christ, so too there was the fulness of all knowledge, as is plain from what has been said above (7, 9; 9). Now as the fulness of grace and virtue in Christ excluded the "fomes" of sin, so the fulness of knowledge excluded ignorance, which is opposed to knowledge. Hence, even as the "fomes" of sin was not in Christ, neither was there ignorance in Him.
Reply to Objection 1. The nature assumed by Christ may be viewed in two ways. First, in its specific nature, and thus Damascene calls it "ignorant and enslaved"; hence he adds: "For man's nature is a slave of Him" (i.e. God) "Who made it; and it has no knowledge of future things." Secondly, it may be considered with regard to what it has from its union with the Divine hypostasis, from which it has the fulness of knowledge and grace, according to John 1:14: "We saw Him [Vulgate: 'His glory'] as it were the Only-begotten of the Father, full of grace and truth"; and in this way the human nature in Christ was not affected with ignorance.
Reply to Objection 2. Christ is said not to have known sin, because He did not know it by experience; but He knew it by simple cognition.
Reply to Objection 3. The prophet is speaking in this passage of the human knowledge of Christ; thus he says: "Before the Child" (i.e. in His human nature) "know to call His father" (i.e. Joseph, who was His reputed father), "and His mother" (i.e. Mary), "the strength of Damascus... shall be taken away." Nor are we to understand this as if He had been some time a man without knowing it; but "before He know" (i.e. before He is a man having human knowledge)--literally, "the strength of Damascus and the spoils of Samaria shall be taken away by the King of the Assyrians"--or spiritually, "before His birth He will save His people solely by invocation," as a gloss expounds it. Augustine however (Serm. xxxii de Temp.) says that this was fulfilled in the adoration of the Magi. For he says: "Before He uttered human words in human flesh, He received the strength of Damascus, i.e. the riches which Damascus vaunted (for in riches the first place is given to gold). They themselves were the spoils of Samaria. Because Samaria is taken to signify idolatry; since this people, having turned away from the Lord, turned to the worship of idols. Hence these were the first spoils which the child took from the domination of idolatry." And in this way "before the child know" may be taken to mean "before he show himself to know."
Source: Summa Theologica (New Advent)