Bede

And there appeared to them Elijah with Moses, and they were talking with Jesus. Moses and Elijah, one of whom we read was dead and the other taken up into the heavens, were seen in majesty with the Lord (as Luke writes), signifying the future glory of all the saints in Him. Evidently at the time of judgment, some will be found alive in the flesh, or also, having previously tasted death, will be resurrected and will reign together with Him. As the Apostle testifies: The dead in Christ will rise first; then we who are alive, who are left, will be caught up together with them to meet the Lord in the air, and so we will always be with the Lord (1 Thess. IV). Alternatively, Moses and Elijah, that is, the legislator and the eminent prophet, appear and speak with the Lord coming in the flesh, to show that He is the one whom all the oracles of the law and the prophets promised. However, they appear not in the lowly places, but on the mountain with Him, because, indeed, only those who transcend earthly desires in their mind perceive the majesty of Holy Scripture, which is fulfilled in the Lord. Finally, even the sons of Israel saw Moses, but they did not deserve to follow him as he went up to God on the mountain, and they saw him turned back to them not without a veil. They knew Elijah, but it was only Elisha who with the sons of the prophets contemplated his triumphant ascent. Because many of us widely read the words of Scripture; but how exalted it shines in the mysteries of Christ, very few more perfect ones understand.