128 But if this moves you that He said “searches,” learn that this is also said of God, inasmuch as He is the searcher of hearts and reins. For Himself said: “I am He that searches the heart and reins.” And of the Son of God you have also in the Epistle to the Hebrews: “Who is the Searcher of the mind and thoughts.” Whence it is clear that no inferior searches the inward things of his superior, for to know hidden things is of the divine power alone. The Holy Spirit, then, is a searcher in like manner as the Father, and the Son is a searcher in like manner, by the proper signification of which expression this is implied, that evidently there is nothing which He knows not, Whom nothing escapes.
129. Lastly, he was chosen by Christ, and taught by the Spirit. For as he himself witnesses, having obtained through the Spirit knowledge of the divine secrets, he shows both that the Holy Spirit knows God, and has revealed to us the things which are of God, as the Son also has revealed them. And he adds: “But we received, not the spirit of this world, but the Spirit which is of God, that we might know the things that are given to us by God, which we also speak, not in persuasive words of man's wisdom, but in manifestation of the Spirit and in the power of God.”
Source: On the Holy Spirit (New Advent)