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http://www.newadvent.org/summa/4010.htm#article2
<Arius and Eunomius understood this saying, not of the knowledge of the soul, which they did not hold to be in Christ, as was said above (III:9:1), but of the Divine knowledge of the Son, Whom they held to be less than the Father as regards knowledge. But this will not stand, since all things were made by the Word of God, as is said John 1:3, and, amongst other things, all times were made by Him. Now He is not ignorant of anything that was made by Him.
<He is said, therefore, not to know the day and the hour of the Judgment, for that He does not make it known, since, on being asked by the apostles (Acts 1:7), He was unwilling to reveal it; and, on the contrary, we read (Genesis 22:12): "Now I know that thou fearest God," i.e. "Now I have made thee know." But the Father is said to know, because He imparted this knowledge to the Son. Hence, by saying but the Father, we are given to understand that the Son knows, not merely in the Divine Nature, but also in the human, because, as Chrysostom argues (Hom. lxxviii in Matth.), if it is given to Christ as man to know how to judge—which is greater—much more is it given to Him to know the less, viz. the time of Judgment. Origen, however (in Matth. Tract. xxx), expounds it of His body, which is the Church, which is ignorant of this time. Lastly, some say this is to be understood of the adoptive, and not of the natural Son of God.