Numbers 31:23
Every thing that may abide the fire, ye shall make it go through the fire, and it shall be clean: nevertheless it shall be purified with the water of separation: and all that abideth not the fire ye shall make go through the water. Every thing that may abide the fire, ye shall make it go through the fire, and it shall be clean: nevertheless it shall be purified with the water of separation: and all that abideth not the fire ye shall make go through the water.
The command governed the cleansing of the articles taken as spoil in Israel's war against Midian. Everything that could abide fire was to be made to pass through fire so that it would be clean, yet even then it was still to be purified with the water of separation. Everything that could not abide fire was instead to be made to pass through water. That same cleansing standard applies to God's people, who must be clean in person and pure in heart in order to be presented blameless to God.