Ante-Nicene Fathers, Vol IX:
Origen's Commentary on the Gospel of Matthew.: Chapter XXXIX

39.  Jesus Was Transfigured—“As He Was Praying.”

Then, since it will be necessary to expound the passage as given in Mark, “And as He was praying He was transfigured before them,” 5788 we must say that perhaps it is possible especially to see the Word transfigured before us if we have done the things aforesaid, and gone up into the mountain, and seen the absolute Word holding converse with the Father, and praying to Him for such things as the true High-Priest might pray for to the only true God.  But in order that He may thus hold fellowship with God and pray to the Father, He goes up into the mountain; and then, according to Mark, “His garments become white and glistening as the light, so as no fuller on earth can whiten them.” 5789   And perhaps the fullers p. 471 upon the earth are the wise men of this world who are careful about the diction which they consider to be bright and pure, so that even their base thoughts and false dogmas seem to be beautified by their fulling, so to speak; but He who shows His own garments glistering to those who have ascended and brighter than their fulling can make them, is the Word, who exhibits in the expressions of the Scriptures which are despised by many the glistering of the thoughts, when the raiment of Jesus, according to Luke, becomes white and dazzling. 5790

Footnotes

Luke 9:28, 29 alone mentions the praying.

Mark ix. 3.

Luke ix. 29.