He returns to the conversion of the Gentiles, and shows that it was foretold
And when all had given assent, I said:
Justin: I would now adduce some passages which I had not recounted before. They are recorded by the faithful servant Moses in parable, and are as follows: 'Rejoice, O you heavens, with Him, and let all the angels of God worship Him;'
And I added what follows of the passage:
'Rejoice, O you nations, with His people, and let all the angels of God be strengthened in Him: for the blood of His sons He avenges, and will avenge, and will recompense His enemies with vengeance, and will recompense those that hate Him; and the Lord will purify the land of His people.' And by these words He declares that we, the nations, rejoice with His people—to wit, Abraham, and Isaac, and Jacob, and the prophets, and, in short, all of that people who are well-pleasing to God, according to what has been already agreed on between us. But we will not receive it of all your nation; since we know from Isaiah that the members of those who have transgressed shall be consumed by the worm and unquenchable fire, remaining immortal; so that they become a spectacle to all flesh.
But in addition to these, I wish, sirs, to add some other passages from the very words of Moses, from which you may understand that God has from of old dispersed all men according to their kindreds and tongues; and out of all kindreds has taken to Himself your kindred, a useless, disobedient, and faithless generation; and has shown that those who were selected out of every nation have obeyed His will through Christ,—whom He calls also Jacob, and names Israel—and these, then, as I mentioned fully previously, must be Jacob and Israel. For when He says, 'Rejoice, O you nations, with His people,' He allots the same inheritance to them, and does not call them by the same name; but when He says that they as Gentiles rejoice with His people, He calls them Gentiles to reproach you. For even as you provoked Him to anger by your idolatry, so also He has deemed those who were idolaters worthy of knowing His will, and of inheriting His inheritance.
Source: Dialogue with Trypho (New Advent)