The Holy Spirit is a witness to the life and saving power of Jesus

Acts 5:30-32

The God of our fathers raised up Jesus, whom you killed, hanging him on a tree. God exalted him with his right hand to be a Prince and a Savior, to give repentance to Israel, and remission of sins. We are his witnesses of these things; and so also is the Holy Spirit, whom God has given to those who obey him.