John Chrysostom

"But He that searcheth the hearts." You see that it is not about the Comforter that he is speaking, but about the spiritual heart. Since if this were not so, he ought to have said, "He that searcheth" the Spirit. But that thou mayest learn that the language is meant of a spiritual man, who has the gift of prayer, he proceeds, "And he that searcheth the hearts knoweth what is the mind of the Spirit," that is, of the spiritual man. …

Homily on Romans 14 ↗

Theophylact of Ohrid

That man, he says, stands and prays not because God is unaware of our needs. He who searches the hearts knows "what the mind of the Spirit is," that is, of the spiritual man; He intercedes on behalf of the believers who are in the church, for they are saints, namely for what is pleasing to God. And this happens so that we may learn to ask "according to the will of God," that is, what is pleasing to God. Therefore, if the Spirit intercedes for us, do not grieve when you are afflicted.

Commentary on Romans ↗

Thomas Aquinas

Then he shows the efficacy of the help with which the Holy Spirit aids us, saying: "he who searches the hearts," i.e., God, who alone searches the heart: "you who tried the minds and hearts" (Ps 7:9). But God is said to search hearts, not as though he investigates the secrets of the heart, but because he knows clearly the hidden things of the heart: "I will search Jerusalem with lamps" (Zeph 1:12). …

Commentary on Romans ↗