He now says the same thing of the children of Ammon, as he said before of the Moabites, that some hope yet remained for them, for God would at length show mercy to that nation. But, as we have said, these promises were but adventitious, because God had chosen but one people to be a Father to them; and the children of Abraham must be viewed as distinct from all other nations. But though God built, as it were, a wall to separate his people from aliens, it was yet his will to give some preludes of his favor, and of the calling of the Gentiles. The Prophet, then, had here a regard to the kingdom of Christ. The promise, no doubt, extended itself to his coming; for he speaks of the calling of the Gentiles, which God deferred until he manifested his own Son to the world. It is the same then, as though the Prophet had said, that God's mercy would at length be showed to the Ammonites in common with others; that is, when God would gather his Church from the whole world, and unite, in one body, those who were before scattered. Nor is there a doubt but that the Prophet, speaking of the children of Ammon, intended to show what was to be manifested through all parts of the world. And so it is, that on our calling is our salvation founded, for we see that the gospel has not been, without a design, proclaimed to the world; but as God had determined and settled this from the beginning, so we see that Jeremiah was a herald of our adoption. This, then, is the import of what is said. He afterwards passes over to the children of Edom. But I cannot now proceed farther.

In 1 Macc. 5:6, 7, the Ammonites appear again as a powerful nation.

Afterward I will bring again - The Ammonites are supposed to have returned with the Moabites and Israelites, on permission given by the edict of Cyrus.

And (h) afterward I will bring again the captives of the children of Ammon, saith the LORD.
(h) In the time of Christ, when the Gentiles will be called.

And afterwards I will bring again the captivity of the children of Ammon, saith the Lord. Perhaps by Cyrus; for, in the times of Judas Maccabeus, the children of Ammon were again a large and mighty people,
"Afterward he passed over to the children of Ammon, where he found a mighty power, and much people, with Timotheus their captain.'' (1 Maccabees 5:6)
Justin Martyr (f) says, that in his time there was a large multitude of Ammonites; but Origen (g), who was later than he, observes, that not only the Idumeans, but the Ammonites and Moabites, were then called by the common name of Arabians; and these are now the present inhabitants of their country; and when these shall be converted in the latter day; see Isaiah 60:6; who may be called by the name of the ancient inhabitants; then will this be more fully accomplished: for some refer this to the days of the Messiah, and to the conversion of some of these Heathen people, either in the first times of the Gospel, or in the latter day; See Gill on Jeremiah 49:2. The Jews (h) understand this as fulfilled in Ammonite proselytes to their religion.
(f) Dialog. cum Tryphone Judaeo, p. 347. (g) Comment. in lib. Job, fol. 2. 1. A. (h) Misn. Yadaim, c. 4. sect. 4. T. B. Beracot, fol. 28. 1.

(Compare Jeremiah 48:47). For the sake of "righteous" Lot their progenitor. Partially fulfilled under Cyrus; in gospel times more fully.

Yet afterwards, the fortunes of Ammon also shall be changed, as it was with Moab. Jeremiah 48:47.
Regarding the fulfilment of this prophecy (just as in the case of Moab), we have no further information than that of Josephus (Ant. x. 9. 7), that Nebuchadnezzar defeated and subdued the Ammonites in the fifth year after the destruction of Jerusalem. Shortly before, their king Baalis had got Gedaliah the governor put out of the way (Jeremiah 40:14). Even after the exile they kept up their hostile spirit against the Israelites and the Jews, inasmuch as they tried to hinder the building of the city walls at Jerusalem (Nehemiah 4:1.), and in the Maccabean age were still making war against the Jews; 1 Macc. 5:6, 30-43. Their name was preserved till the time of Justin Martyr ( ̓Αμμανιτῶν ἐστι νῦν πολὺ πλῆθος, Dial. Tryph. p. 272). But Origen already comprehends their country under the general name Arabia (lib. 1 in Jobum).

I will bring - Probably this refers to the conversion of the Ammonites, as well as other Heathens, to Christ.