Lapide:
The Spirit searcheth all things, the deep things of God. That is, penetrates into and perceives everything. For when men want to learn something of which they are ignorant, they are wont to search and inquire about it. But God, without any such searching, knows everything at a glance, and as it were by a single application of His mind. (S. Thomas, Theodoret, Theophylact.)
The deep things of God are all the most secret and inward counsels of God. Amongst them the chiefest is this mystery of man’s glory and redemption by Christ. All these the Holy Spirit penetrates into and clearly views, because He is of one essence and knowledge with God, and therefore He so “searches the deep things of God,” that nothing in God remains unknown t Him. His knowledge and sight equal their object, and He knows God as he can be known; i.e., the Holy Spirit, because He is God, comprehends God and His Divinity as completely as He comprehends Himself. (Molina part i. qu. 14, a. 3, Theodoret, S. Thomas.) From this passage Ambrose and other Fathers prove the Godhead of the Holy Ghost against the Macedonians. To sum up S. Paul’s meaning: The Holy Spirit has revealed to us these mysteries and secrets of God: He knows all the secrets of God, and therefore He searches and clearly views the deep things of God.
http://www.catholicapologetics.info/scr ... stcor2.htm
