Midrash su Isaia 54:15

Another matter, “all the rivers go to the sea,” all the converts enter only into Israel. “Yet the sea is not full,” and Israel’s number is never lacking,⁴⁵Their number is not filled out by the converts, and they will not be lacking for numbers without converts. as it is stated: “Who has counted the dust of Jacob” (Numbers 23:10). Perhaps you will say that anyone who does not convert in this world will convert in the World to Come; the verse states: “Behold, they may gather together [gor yagur]” (Isaiah 54:15),⁴⁶The reference is to converts [gerim], who can gather together now, but not in the future. but from then on “but not by Me [efes meoti]” (Isaiah 54:15) – stop from being with Me [hafsek me’iti]. Who is with Me? He “who gathers to You” (Isaiah 54:15) in this world “will fall in with You” (Isaiah 54:15) in the World to Come. Rabbi Yoḥanan said: Moreover, in the place where one converts, from there he takes his portion, as it is stated: “It will be that in the tribe where the stranger resides, [there you shall give him his inheritance, said the Lord God]” (Ezekiel 47:23).

Another matter, “all the rivers go to the sea,” – all the dead enter only the grave, but the grave is never filled, as it is stated: “The grave and oblivion are not sated…” (Proverbs 27:20). You might say that once they die in this world they do not live again in the World to Come; the verse [therefore] states: “To the place that the rivers go, they go there again” – to the place that the dead gather in the World to Come, there they return and are destined to recite songs in the messianic era. What is the reason?⁴⁷What is the source? “From the ends of the earth we have heard songs” (Isaiah 24:16); “your dead will live, my corpses shall arise” (Isaiah 26:19).