From The Gospel of Matthew by Curtis Mitch and Edward Sri, commenting on Matthew 28:5-8:
With such a display of heavenly majesty, it is no surprise that the angel’s first words to the women are: Do not be afraid! The angel knows that the women intend to pay their last respects to the crucified Messiah. But they will not get the chance. He is not here, declares the angel, for he has been raised. The power of death, which holds the human race captive, has been overpowered. And this is just what Jesus said. His passion predictions, which included reference to his rising from the grave, have come true (16:21; 17:23; 20:19). This is the joyful proclamation that stands at the center of the Christian message (see Acts 4:2; 1 Pet 1:3). Should there be any doubt about its truth, the women are invited to peer into the tomb and see the vacant slab where his body had once lain.
Attached to the angel’s announcement are instructions for the disciples. The women are to be “apostles to the apostles,” the first to bear the message of the resurrection to others.