The Holy Spirit "testifies" Christ to the heart of man
The Holy Spirit shows man Christ. The Holy Spirit reveals the person of Christ. Christ is made known to the human family by the ministry of the Holy Spirit. This is the witness of God to the human family.
When the Gospel is preached Christ is made known. Christ shows the sinner his need to turn from sin and disobedience. Light shines and reveals sin. The Spirit's testimony tells the sinner there is a better way in the person of Christ.
This witness of the Spirit can convict. This witness can stir man so much that fallen man will turn from evil and wrongdoing. The Spirit's witness can be seen in goodness that "leads" or "causes" fallen man to change his mind. The Gospel message gets into man's thinking enough that the Holy Spirit can sway fallen man to believe.
The Holy Spirit "testifies" Christ to the human soul. There is power in preaching because the ministration of the Holy Spirit is behind preaching and touches man with truth and love. This touch is the Spirit's testimony of Christ.
John 15:26-27... But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me: And ye also shall bear witness