Immediately before Moshe passed away, he renewed the covenant between God and B’nai Yisrael which was originally established with Avraham, Yitzchak and Yaakov.
In Parshat Nitzavim, Dvarim 29:13-14 we find an interesting concept:
Not with you alone do I seal this covenant and this imprecation, but with whoever is here, standing with us today before Hashem, our God with whoever is not here with us today.
How can a covenant be binding on those who have not yet been born?
According to Rabbeinu Bachya, the trunk of the tree is viewed as the father, “the root” of all subsequent branches. All the branches are only the offshoots of the original roots. Concerning how it is possible to include unborn generations in this covenant, our sages in Tanchuma Yitro 11 comment that all souls ever created since the days of the creation of the earth were present at this assembly. They are not all described as “standing,” as souls without bodies cannot “stand.” The words: “who are not here,” refer to the bodies of such souls. The words “with us this day”, refer to their souls, which are present even today. The words: “with us,” refers back to the word “standing,” in verse 14. It is as if the Torah had written: “and all those not standing here with us this day.” The reason Moshe uses the word “standing,” is because the angels in Zecharyah 3:7 were described as “among these who are standing.”
In Midrash Tanchuma, our sages claim that all the prophets who ever lived after Moshe received their prophetic powers when their souls were at the assembly at Sinai. However, they were not allowed to use these powers until their respective generations when they were alive on earth. This is also the meaning of Malachi 1:1: “The burden of the word of God to Yisrael by Malachi”. It is significant that the wording is “by Malachi”. The present formulation means: “A pronouncement: ‘the word of God to Israel through Malachi’” rather than the message only now coming to Malachi. The present formulation describes only the point in time when this message, which the soul of the prophet had received at Mount Sinai, was released for publication. We find something similar in Yishayahu 48:16 where the prophet writes: “from the beginning I did not speak in secret; from the time anything existed I was there. And now, the Lord, God has sent me, endowed me with His spirit.” The meaning of the verse is that the soul of the prophet had received this communication from God already at Mount Sinai but it had only now been released for publication.
We see from here that the Jewish souls were present at the covenant in Parshat Nitzavim, just like they were all at Mt. Sinai when B’nai Yisrael received the Torah.