There are different ways to be a witness. Usually we understand this word to refer to a person who sees an event, and is able to recount what happened, as in a witness at a trial. We can also understand that a witness as someone who give an example to some belief, as in someone who witnesses to the truth by the way in which they live. We do not have the experience in our own lives to have seen the events that occurred after the resurrection of Jesus. But we do have those experiences where we can witness to our experiences of the risen Christ.
And those experiences may not be that much different than those experienced shortly after the resurrection of Jesus. “Jesus the Nazarene was a man commended to you by God with mighty deeds, wonders, and signs, which God worked through him in your midst.” These mighty deeds, wonders and signs of God are still being poured out in our midst today.