The Son of Bethera said: Moses spent forty days on the mount, expounding the meaning of the words of the Torah, and examining its letters. After forty days he took the Torah, and descended on the tenth of the month, on the Day of Atonement, and gave it as an everlasting inheritance to the children of Israel, as it is said, "And this shall be unto you an everlasting statute" (Lev. 16:34).
Rabbi Zechariah said: They read in the Torah and found written therein, "And ye shall afflict your souls" (Lev. 16:29), and on the Day of Atonement they caused a Shophar to be sounded throughout all the camp and proclaimed a fast for all Israel, old and young. Were it not for the Day of Atonement the world could not stand, because the Day of Atonement is in this world and in the world to come, || as it is said, "It is a sabbath of sabbaths unto you" (Lev. 16:31). "A sabbath" refers to this world, "sabbaths" refers to the world to come. Moreover, if all the festivals pass away, the Day of Atonement will not pass away, for the Day of Atonement effects reconciliation for serious offences as well as for slight offences. Whence do we know that the Day of Atonement effects reconciliation? Because it is said, "For on this day shall atonement be made for you, to cleanse you; from all your sins shall ye be clean" (Lev. 16:30). "From your sins" is not written here, but "from all your sins shall ye be clean before the Lord" (ibid.).