The idea derives from the Torah, where Moses speaks of being inscribed in (or erased from) the book God has written (Ex. 32:32-33).
In the Talmud (RH 16b), Rabbi Kruspedai says in the name of Rabbi Yochanan that God has three books, one for the perfectly righteous, one for the totally wicked, and one for the in-between category.
The fate of the first two types of person is recorded without delay. The book of the intermediate category is left open during the Ten Days of Penitence before God makes His decision about them.
Since Rabbi Yochanan is no fool, he must be telling us something significant in this passage. The message is that the majority of people come in the intermediate category.
Someone said, “God must love ordinary people, since He made so many of them”. Very interesting, but why does God leave us in limbo while He makes up His mind about us?
Because, as the prayer book declares, “God waits to the very last moment for a person to repent”. Until the eleventh hour and even later we are still able to move from the “wicked” category towards the “righteous” one.
The decisions we make in the week leading to Yom Kippur can be life-changing. In the atmosphere of these days a sudden thought can enter our minds, even a lone word, which sets us off in a new direction.
A husband told me that when his wife remarked, “Our marriage is about us, not just the children”, he suddenly knew what he had to do.