כִּ֣י מַרְבִּ֣ית הָעָ֡ם רַ֠בַּת מֵֽאֶפְרַ֨יִם וּמְנַשֶּׁ֜ה יִשָּׂשכָ֤ר וּזְבֻלוּן֙ לֹ֣א הִטֶּהָ֔רוּ כִּֽי־אָכְל֥וּ אֶת־הַפֶּ֖סַח בְּלֹ֣א כַכָּת֑וּב כִּי֩ הִתְפַּלֵּ֨ל יְחִזְקִיָּ֤הוּ עֲלֵיהֶם֙ לֵאמֹ֔ר יְהוָ֥ה הַטּ֖וֹב יְכַפֵּ֥ר בְּעַֽד׃
For a multitude of the people, even many of Ephraim and Manasseh, Issachar and Zebulun, had not cleansed themselves, yet did they eat the passover otherwise than it is written. For Hezekiah had prayed for them, saying: ‘The good LORD pardon
Read Kabbalistic (mystical) interpretations of II Chronicles 30:18: Zohar and classical Jewish mystical sources, free online.