Great is the mitzva of procreation, for through it Israel was redeemed from Egypt, as it says: “The Israelites were fertile and prolific; they multiplied and increased very greatly, so that the land was filled with them…. The more they were oppressed, the more they increased and spread out” (Shemot 1:7, 12). The Sages said that there will never be a generation of Israel with fewer than 600,000 people. It was only when Israel reached that number that they became a nation and could leave Egypt and receive the Torah (Zohar, Ra’aya Mehemna III 216b; Tiferet Yisrael ch. 29). Had they not made efforts to fulfill the mitzva, had they been even one person short, they would not have merited leaving Egypt or receiving the Torah (Devarim Rabba 7:8). That is what the Sages meant when they said, “In the merit of the righteous women of that generation, Israel was redeemed from Egypt” (Sota 11b). In reward for their efforts to ensure continuity, they raised “all this proliferation” (Tanḥuma Pekudei §9; above, 1:8).
It is important to add that after the Holocaust, in which we lost six million of our people, it is a greater mitzva than ever to have families which are as large as possible. We must recreate what was destroyed, so that the Jewish people can realize its divine mission (see Ramban on Devarim 30:2).
The Sages say that just as Israel was redeemed from Egypt because they procreated, so too they will be redeemed in the future because of their procreation, as it says (Yeshayahu 54:3), “For you shall spread out to the right and the left; your offspring shall dispossess nations and shall people the desolate towns” (Eliyahu Zuta 14).
Likewise, R. Assi states, “The (messianic) son of David will not arrive until all the souls of the body have been finished, as it says (Yeshayahu 57:16), ‘For I will not always contend; I will not be angry forever. For the spirit that enwraps itself is from Me; it is I Who made souls’” (Yevamot 62a). This means that there is a storehouse of Jewish souls, and each Jew who is born reveals a spark from it. Even if a baby dies immediately after birth, its soul reveals something in this world and brings the redemption nearer. Thus, even if the Jewish people do not repent properly (God forbid), once all the souls are born, all necessary correctives will be achieved. The process of redemption will advance, and as a result the Jewish people will repent. If, instead, we wake up earlier and repent fully, God will speed up the redemption. First, every woman will have the privilege of giving birth to more children (Tosafot to Nidda 13b, s.v. “ad”). Second, every Jew who is born will reveal many sparks rather than just one (Maharsha, Nidda ad loc.). This will use up all the souls in the warehouse, so the redemption will arrive speedily.
Thus, by fulfilling the mitzva of procreation we draw the redemption nearer, and through its blessings the redemption will be realized, as it is written:
I myself will gather the remnant of My flock from all the lands to which I had banished them, and I will bring them back to their pasture, where they will be fruitful and multiply. (Yirmiyahu 23:3)
Likewise, it says:
You, O mountains of Israel, shall yield your produce and bear your fruit for My people Israel, for their return is near. For I will care for you; I will turn to you, and you shall be tilled and sown. I will settle a large population on you – the whole House of Israel; the towns shall be resettled, and the ruined sites rebuilt. I will multiply men and beasts upon you, and they will multiply and be fruitful, and I will resettle you as you were formerly, and will make you more prosperous than you were at first. And you shall know that I am the Lord. (Yeḥezkel 36:8-11)
Thus said the Lord, God: Moreover, in this I will respond to the House of Israel and act for their sake. I will multiply their people like sheep. As Jerusalem is filled with sacrificial sheep during her festivals, so shall the ruined cities be filled with flocks of people. And they shall know that I am the Lord. (ibid., vv. 37-38)