"Ma'asehavot siman l'vanim – the deeds of our ancestors are a sign for the children."
The Rabbis see paradigms in the biblical narrative. They apply them to explain our lives to us, who suffer the same existential realities as our ancestors.
In Parashat Shelah Lecha 12 spies take upon themselves the task of spying out the land of Israel and reporting back whether they should attempt to conquer it. Although God has repeatedly prescribed that God's people will return to the land founded by Abraham and Sarah and abandoned by Jacob and his children, nonetheless 10 of the spies return an evil report and advise against settlement. God becomes incensed, and tells Moses, "I will strike them with pestilence and disown them; and I will make of you a nation far more numerous than they." (14:12).
From the time of Abraham, God's people have been a demonstration project for the world. After the failures with Adam and Eve and Noah's generation, God selects one family. That family is to act as God's people. When they do right, they will be rewarded. When they transgress, they will be punished. Seeing this, the nations of the earth will learn of the one true God, and abandoning false gods turn to the God of Israel.
But in God's anger, God threatens the entire project that has gone on for at least a half millennium. Moses perceives the error, and responds to God with tact but firmly, "If then You slay this people to a man, the nations who have heard of your family will say, 'It must be because the Lord was powerless to bring that people into the land which He had promised them on oath that He slaughtered them in the wilderness.'" (vss 14-16). Thus, God's investment in humanity would come to a disastrous end.
Moses pleads on behalf of the people. "Therefore, I pray, let my Lord's forbearance be great ... Pardon, I pray, the iniquity of this people according to Your great kindness, as You have forgiven this people ever since Egypt." (vss. 17, 19)
God responds with forgiveness, according to God's nature: "I pardon, as you have asked." (vs. 20)
How does this affect our lives? We all attend worship on Erev Yom Kippur, Kol Nidre worship. The first prayer after Kol Nidre, and therefore technically the first prayer of Yom Kippur (Kol Nidre is prior to Yom Kippur) is, in its entirety, Moses' plea and God's response.
When you and I recite this Bible quotation, or hear the choir sing it (as in Reform congregations), we are saying this:
"Lord, I am about to enter into prayers asking that you forgive my sins. As badly as I have acted this past year, my sins are no greater than your people, my ancestors, who refused the promise of the Land. As you forgave them, please also forgive me after my prayers uttered over the next 25 hours."
Standing before God on Yom Kippur, we place ourselves in God's mind as direct descendants of the people God forgave, and we ask that God extend God's mercy once again, and forgive God's people.
Thus, this parashah constitutes the basis of the Kol Nidre prayers.