Rashi on Leviticus 23:27/רש"י על ויקרא כ״ג:כ״ז

1אך — The words אך and רק wherever they occur in Scripture have a limitative force; here the word implies: it (Yom Kippur) makes expiation for those who repent, but it does not make expiation for those who do not repent (אך... יום הכפורים הוא — in a limitative sense it is Yom Kippur) (Shevuot 13a; cf. Rashi on Exodus 34:7).