“Then the sign
Of the Son of Man
Will appear
In heaven.
Then all the tribes
Of the earth
Will mourn.
They will see
The Son of Man
Coming on
The clouds of heaven
With power
And great glory.”
καὶ τότε φανήσεται τὸ σημεῖον τοῦ Υἱοῦ τοῦ ἀνθρώπου ἐν οὐρανῷ, καὶ τότε κόψονται πᾶσαι αἱ φυλαὶ τῆς γῆς καὶ ὄψονται τὸν Υἱὸν τοῦ ἀνθρώπου ἐρχόμενον ἐπὶ τῶν νεφελῶν τοῦ οὐρανοῦ μετὰ δυνάμεως καὶ δόξης πολλῆς·
This is exactly the same, almost word for word, in Mark, chapter 13:26, and in Luke, chapter 21:27, for the last sentence. The first two sentences, however, are unique to Matthew. Jesus said that the sign of the Son of Man would appear in the heavens (καὶ τότε φανήσεται τὸ σημεῖον τοῦ Υἱοῦ τοῦ ἀνθρώπου ἐν οὐρανῷ). Then all the tribes or races of people on the earth would mourn or lament (καὶ τότε κόψονται πᾶσαι αἱ φυλαὶ τῆς γῆς). They would all see or experience the Son of Man (καὶ ὄψονται τὸν Υἱὸν τοῦ ἀνθρώπου) coming on the clouds of heaven (ἐρχόμενον ἐπὶ τῶν νεφελῶν τοῦ οὐρανοῦ) with his power (μετὰ δυνάμεως) and great glory (καὶ δόξης πολλῆς). The clouds were the common place where theophanies in the Old Testament occurred, as Yahweh often appeared in a cloud on a mountain.