הֲיַהֲפֹ֤ךְ כּוּשִׁי֙ עוֹר֔וֹ וְנָמֵ֖ר חֲבַרְבֻּרֹתָ֑יו גַּם־אַתֶּם֙ תּוּכְל֣וּ לְהֵיטִ֔יב לִמֻּדֵ֖י הָרֵֽעַ׃
Can the Ethiopian change his skin, Or the leopard his spots? Then may ye also do good, That are accustomed to do evil.
Read kabbalistic (mystical) interpretation of Jeremiah 13:23: and classical mystical sources.