I will therefore put you in remembrance, though you once knew this, how that the Lord, having saved the people out of the land of Egypt, afterward destroyed them that believed not.
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Aquinas Study Bible
AD 2017
Jesus having saved the people: He is referring not to Jesus the son of Nun but to our Lord, showing first that he did not have his beginning at his birth, as the heretics have wished to assert, but he existed as the eternal God. (St. Bede) He pitied them when they were calling out to him, but he plunged them afterward into worse ruin when they lost faith in the desert and were murmuring against God. Their slavery in Egypt prefigures our slavery to our former vices. (Erasmus)
"For I would have you know "says Jude, "that God, having once saved His people from the land of Egypt, afterwards destroyed them that believed not; and the angels which kept not their first estate, but left their own habitation, He hath reserved to the judgment of the great day, in everlasting chains under darkness of the savage angels."
But I will admonish you, that once not as man, but as God, having saved the people of Israel from their slavery in Egypt, did afterwards on several occasions punish and destroy those among them, who believed not; who were rebellious and incredulous to his promises. (Witham)
The Greek, and after it the Protestant version, have the Lord saved; the Vulgate has Jesus, which signifies Saviour, and may in this place be understood of the Word, who from his incarnation took the name of Jesus (Bible de Vence)
Menochius says it means Josue.