Acts 22:15

For thou shalt be his witness unto all men of what thou hast seen and heard.

Paul was appointed to be a witness to all people of what he had seen and heard of Jesus. That witness was part of God's call and commission to him, confirmed by Jesus' own charge to make him a minister and a witness to serve and bear witness.

His witness included recounting his conversion experience, including his persecution of believers, the light from heaven, and Jesus revealing Himself to him. Telling that account of his conversion gave his testimony convincing force. In the Holy Spirit's power his witness also proclaimed Jesus as the Son of God and explained the prophecies of His suffering, death, and resurrection.

By God's direction his witness was to be carried to the Gentiles, although he had wanted to labor for his own nation. His testimony would help turn people from darkness to light and from the power of Satan to God.