Gerd Luedemann, The Resurrection of Jesus: History, Experience, Theology (Fortpress Press, 1994), 135.
[There] is no really no longer an appearance tradition in [Matthew 28:18-20], although with his meagre ‘when they saw him’, Matthew is indicating that he wants to relate an Easter story. Rather, what we find here is Easter theology, which forces any vision that happened at the time to the side, or replaces it with words. In this way the scene remains open to the present. So what we have here is almost no longer an appearance but an enthronement of Jesus as Lord of heaven and earth, which here, as often, is directly connected with the resurrection, indeed is even identified with it. (Perhaps here it would be better to term the event an appearance of the enthroned Christ.) So the theme of authority as such does not serve to distinguish the Risen Christ from the earthly Jesus but virtually combines the two. The new element is the universal extension of the authority of the risen and the earthly Christ over heaven and earth. So the special feature is not just the combination of appearance and mission, but that of exaltation and mission to the Gentiles.
The Amateur Exegete 