וְשַׁבְתֶּם֙ וּרְאִיתֶ֔ם בֵּ֥ין צַדִּ֖יק לְרָשָׁ֑ע בֵּ֚ין עֹבֵ֣ד אֱלֹהִ֔ים לַאֲשֶׁ֖ר לֹ֥א עֲבָדֽוֹ׃ (ס)
Then shall ye again discern between the righteous and the wicked, between him that serveth God and him that serveth Him not.
Read kabbalistic (mystical) interpretation of Malachi 3:18: and classical mystical sources.