2 “And after six days He takes with Him Peter and James and John.”
Now another says, “after eight,” not contradicting this writer, but most fully agreeing with him. For the one expressed both the very day on which He spoke, and that on which He led them up; but the other, the days between them only.
But mark thou, I pray you, the severe goodness of Matthew, not concealing those who were preferred to himself. This John also often does, recording the peculiar praises of Peter with great sincerity. For the choir of these holy men was everywhere pure from envy and vainglory.
Having taken therefore the leaders, “He brings them up into a high mountain apart, and was transfigured before them: and His face did shine as the sun, and His raiment was white as the light. And there appeared unto them Moses and Elias talking with Him.”
Wherefore does He take with Him these only? Because these were superior to the rest. And Peter indeed showed his superiority by exceedingly loving Him; but John by being exceedingly loved of Him; and James again by his answer which he answered with his brother, saying, “We are able to drink the cup;” nor yet by his answer only, but also by his works; both by the rest of them, and by fulfilling, what he said. For so earnest was he, and grievous to the Jews, that Herod himself supposed that he had bestowed herein a very great favor on the Jews, I mean in slaying him.
But wherefore does He not lead them up straightway? To spare the other disciples any feeling of human weakness: for which cause He omits also the names of them that are to go up. And this, because the rest would have desired exceedingly to have followed, being to see a pattern of that glory; and would have been pained, as overlooked. For though it was somewhat in a corporeal way that He made the disclosure, yet nevertheless the thing had much in it to be desired.
Wherefore then does He at all foretell it? That they might be readier to seize the high meaning, by His foretelling it; and being filled with the more vehement desire in that round of days, might so be present with their mind quite awake and full of care.
Source: Homilies on the Gospel of St. Matthew (New Advent)