John of Damascus — as excerpted in the Catena Aurea on Luke 9:28-31

“(Orat. de Trans fig. §. 8.) Matthew and Mark indeed say that the transfiguration took place on the sixth day after the promise made to the disciples, but Luke on the eighth. But there is no disagreement in these testimonies, but they who make the number six, taking off a day at each end, that is, the first and the last, the day on which He makes the promise, and that on which He fulfilled it, have reckoned only the intervening ones, but He who makes the number eight, has counted in each of the two days above mentioned. But why were not all called, but only some, to behold the sight? There was only one indeed who was unworthy to see the divinity, namely Judas, according to the word of Isaiah, Let the wicked be taken away, that he should not behold the glory of God. (Isai. 26:10 LXX.) If then he alone had been sent away, he might have, as it were from envy, been provoked to greater wickedness. Henceforward He takes away from the traitor every pretext for his treachery, seeing that He left below the rest of the company of the Apostles. But He took with Him three, that in the mouths of two or three witnesses every word should be established. He took Peter, indeed, because He wished to shew him that the witness he had borne to Him was confirmed by the witness of the Father, and that he was as it were to preside over the whole Church. He took with Him James, who was to be the first of all the disciples to die for Christ; but He took John as the clearest singer of the sacred doctrine, that having seen the glory of the Son, which submits not to time, he might sound forth, In the beginning was the Word. (John 1:1.)”