Mark 9:4

The law and the prophets were signified by Moses and Elias; both bear testimony to the divinity of Jesus Christ's mission, which was effectually to close the old, and open the new dispensation. By the apparitions of these two illustrious personages, we learn also that sometimes, though not often, there is, by the permission of heaven, a certain intercourse between the living and the dead. (Bristow)