Albert Barnes Commentary Acts 22:15

"For thou shalt be a witness for him unto all men of what thou hast seen and heard." — Acts 22:15 (ASV)

For you shall be his witness, etc. As an apostle to testify to all people that the Messiah has come, that he has died, that he has risen, and that he is the Saviour of the world.

Of what you have seen and heard. Of the remarkable proof which has been given to you of the Divine mission and character of the Lord Jesus.