For though thou wash thee with nitre, and take thee much soap, yet thine iniquity is marked before me, saith the Lord GOD. For though thou wash thee with nitre - It should be rendered natar or natron, a substance totally different from our nitre. It comes from the root נתר nathar, to dissolve, loosen, because a solution of it in water is abstersive, taking out spots, etc., from clothes. It is still used in the east for the purpose of washing. If vinegar be poured on it, Dr. Shaw says, a strong effervescence is the immediate consequence, which illustrates Proverbs 25:20 : "The singing of songs to a heavy heart is like vinegar upon natron;" that is, there is no affinity between them; opposition and strife are occasioned by any attempt to unite them.
Thine iniquity is marked before me - No washing will take out thy spots; the marks of thy idolatry and corruption are too deeply rooted to be extracted by any human means.