Tractate Rosh Hashanah (folio 16, column 2) teaches that on the Day of Judgment three ledgers are opened and three groups of souls appear before the Holy One, blessed be He.
The perfectly righteous are written and sealed at once for eternal life. The thoroughly wicked are written and sealed at once for Gehinnom. But there is a third group — the great middle, neither saintly nor lost — and these are not so easily disposed of.
Of the first two groups the prophet Daniel wrote: "Many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt" (Daniel 12:2). But of the intermediate souls, the rabbis turned to the prophet Zechariah, who said in the name of the Holy One: "I will bring the third part through the fire, and will refine them as silver is refined, and will try them as gold is tried; they shall call on my name, and I will hear them" (Zechariah 13:9).
They descend into Gehinnom, they cry and howl for a time, and then they ascend. It was of them that Hannah sang in her prayer at Shiloh: "The Lord killeth, and maketh alive; He bringeth down to she'ol, and bringeth up" (1 Samuel 2:6).
Most of us, the rabbis are whispering, will be refined rather than destroyed. That is a strict mercy.