On the mount of Transfiguration, Peter was so awestruck when he saw Moses and Elijah appear and talk with Jesus that he blurted out, "Rabbi, it is good for us to be here; and let us make three tabernacles, one for You, one for Moses, and one for Elijah" (Mark 9:5). Jesus said nothing, but "then a cloud formed, overshadowing them, and a voice came out of the cloud, "This is my beloved Son, listen to Him!" And all at once they looked around and saw no one with them anymore, except Jesus alone(vv. 7-8). God was saying to Peter—and to James and John and every person—"Don't listen to Moses and Elijah. This is My Son; hear Him and Him alone." Even before the New Covenant was finalized by Jesus' crucifixion and resurrection, God illustrated that the Old had passed. The covenant represented by Moses and Elijah was gone (cf. John 4:21-24).
At Sinai the people were fenced off at the foot of the mountain, so they could not approach God. In the Tabernacle and in the Temple the veil stood between them and God's presence in the Holy of Holies. The Old Covenant not only did not bring men into God's presence, it forbade them from trying to get there. Without full cleansing, complete forgiveness of sins, they were not qualified. But Jesus, so to speak, came down the mountain to the people and tore down the veil.
So the whole Judaistic system was changed—not just changed, but exchanged—for a new order, a new Priest, a new sacrifice, an entirely New Covenant."