The same holy apostle had said in a similar way, “No one knows the thoughts of a person except the spirit of the person that is in him. So also no one comprehends the thoughts of God.” In this second expression he does not add “except the spirit of the person that is in him.” He limits himself to adding “except the Spirit of God,” because one does not think of the divine Being as though he were a composite of two parts put together. He says “except the Spirit of God” also because “it is the Spirit of God who probes everything, even the depths of God.” We therefore do not say that his Spirit, who also probes the depths of God, is a stranger to God. Holy Scripture cautions us, “Reflect on what has been assigned to you, for you do not need what is hidden,” or, “Seek not what is too difficult for you; nor investigate what is beyond your power.” Of course, we cannot apply words that condemn vain curiosity to the Holy Spirit. We should say only that it is part of his nature to investigate the hidden things of God.
Source: ANCORATUS 12:1-4