1 Cor 2:1-5
"And I, when I came to you, brothers, did not come proclaiming to you the testimony of God with lofty speech or wisdom... so that your faith might not rest in the wisdom of men but in the power of God."
Paul deliberately renounced rhetorical eloquence so that the faith of the Corinthians would not rest on human ability but on the power of God. The demonstration (apodeixis) of the Spirit is the evidence of transformed lives.
1 Cor 2:9-12
"But, as it is written: What no eye has seen... are the things God has prepared for those who love him. But God has revealed them to us through his Spirit."
The Spirit (pneuma) is the agent of revelation — he knows the depths of God (ta bathe tou theou) just as the human spirit knows the depths of the human being.