Beginning at Jerusalem

This is how Luke tells the commission of the risen Jesus. In Acts, his second book, he picks up this thread again. Look at where it has to begin. In Jerusalem Jesus had just been condemned and crucified. It is precisely there that forgiveness is offered first.

And the disciples are called witnesses, in Greek martys. Our word martyr comes from it, but that meaning only came later. A witness doesn't have to explain or prove everything. He tells what he has seen. You don't need to be a scholar for that.