John Chrysostom

He brings Moses and Elias before them; first, indeed, because the multitudes said that Christ was Elias, and one of the Prophets, He shows Himself to the Apostles with them, that they might see the difference between the Lord, and His servants. …

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

For many reasons Elijah and Moses appear conversing with Christ. But it suffices to mention two. First, since the disciples were pleased that among the people some took Him for Elijah and others for one of the prophets, He shows them the greatest prophets, so that the disciples might at least in this way perceive the difference between servants and the Master. This is the first reason. …

Commentary on Mark ↗

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. …

On the Gospel of Mark ↗