The earliest Patristic reference I’ve found so far is from St. Ambrose, who appears to “spiritualize” the promise:
- Paul certainly is dead, and by his honourable passion exchanged the life of the body for everlasting glory; did he then deceive himself when he wrote that he should be caught up alive in the clouds to meet Christ? We read the same too of Enoch and of Elijah,12 and thou too shalt be caught up in the Spirit. Lo the chariot of Elijah, lo the fire, though not seen are prepared, that the just may ascend, the innocent be borne forth, and thy life may not know death.** For indeed the apostles knew not death, according to that which was said: “Verily, verily, I say unto you, many of those standing here shall not taste death until they see the Son of man coming in His kingdom.” **For he lives, who has nothing in him which can die, who has not from Egypt any shoe or bond, but has put it off before laying aside the service of this body. And so not Enoch alone is alive, for not he alone was caught up; Paul also was caught up to meet Christ.
- The patriarchs also live, for God could not be called the God of Abraham, of Isaac, and of Jacob, except the dead were living; for He is not the God of the dead but of the living. And we, too, shall live if we be willing to copy the deeds and habits of our predecessors. We are astonished at the rewards of the patriarchs, let us copy their faithfulness; we tell of their grace, let us follow their obedience; let us not, enticed by appetite, fall into the snares of the world.
Ambrose of Milan, “The Two Books on the Decease of His Brother Satyrus,” in St. Ambrose: Select Works and Letters, ed. Philip Schaff and Henry Wace, trans. H. de Romestin, E. de Romestin, and H. T. F. Duckworth, vol. 10, A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series (New York: Christian Literature Company, 1896), 189.
Ambrose elsewhere treats the verse as evidence of unity between the kingdoms of the Father and the Son–I’m not sure, but there may be an element of realized eschatology here, perhaps referring to the Transfiguration as a revelation of the Son’s kingdom on earth, in contrast to the heavenly kingdom of the Father:
- To begin with, learn, from further testimonies [of Scripture], how that the kingdom of heaven is also the kingdom of the Son:** “Verily, verily, I say unto you, that there are some amongst those which stand here with us, who shall not taste death, until they see the Son of Man coming into His kingdom.”** There is therefore no room for doubt that the kingdom appertaineth to the Son of God.
- Now learn that the kingdom of the Son is the very same as the kingdom of the Father: “Verily, I say unto you that there be some of those which stand around us, who shall not taste death until they see the kingdom of God coming in power.” So far, indeed, is it one kingdom, that the reward is one, the inheritor is one and the same, and so also the merit, and He Who promises [the reward].
Ambrose of Milan, “Exposition of the Christian Faith,” in St. Ambrose: Select Works and Letters, ed. Philip Schaff and Henry Wace, trans. H. de Romestin, E. de Romestin, and H. T. F. Duckworth, vol. 10, A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series (New York: Christian Literature Company, 1896), 256.
Finally, Leo the Great seems to think that the verse refers to the Transfiguration:
In order, therefore, that the Apostles might entertain this happy, constant courage with their whole heart, and have no tremblings about the harshness of taking up the cross, and that they might not be ashamed of the punishment of Christ, nor think what He endured disgraceful for themselves (for the bitterness of suffering was to be displayed without despite to His glorious power), Jesus took Peter and James and his brother John, and ascending a very high mountain with them apart, showed them the brightness of His glory; because, although they had recognised the majesty of GOD in Him, yet the power of His body, wherein His Deity was contained, they did not know.
And, therefore, rightly and significantly, had He promised that certain of the disciples standing by should not taste death till they saw “the Son of Man coming in His Kingdom,” that is, in the kingly brilliance which, as specially belonging to the nature of His assumed Manhood, He wished to be conspicuous to these three men. For the unspeakable and unapproachable vision of the Godhead Itself, which is reserved till eternal life for the pure in heart, they could in no wise look upon and see while still surrounded with mortal flesh. The LORD displays His glory, therefore, before chosen witnesses, and invests that bodily shape which He shared with others with such splendour, that His face was like the sun’s brightness and His garments equalled the whiteness of snow.
Leo the Great, “Sermons,” in Leo the Great, Gregory the Great, ed. Philip Schaff and Henry Wace, trans. Charles Lett Feltoe, vol. 12a, A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series (New York: Christian Literature Company, 1895), 163.