Ambrose of Milan
Catena Aurea · Lukasevangelium 9
And it came to pass about an eight days after these sayings, he took Peter and John and James, and went up into a mountain to pray. And as he prayed, the fashion of his countenance was altered, and his raiment was white and glistering. And, behold, there talked with him two men, which were Moses and Elias: Who appeared in glory, and spake of his decease which he should accomplish at Jerusalem.
I should think that in the three who are taken up into the mountain, was contained in a mystery the human race, because from the three sons of Noah sprung the whole race of man; I did not perceive that they were chosen out. Three then are chosen to ascend the mountain, because none can see the glory of the resurrection, but they who have preserved the mystery of the Trinity with inviolable purity of faith.