23 everything that can stand the fire, you shall pass through the fire, and it shall be clean. Nevertheless, it shall also be purified with the water for impurity. And whatever cannot stand the fire, you shall pass through the water.
The Israelites had to purify themselves according to the law, and to abide without the camp seven days, though they had not contracted any moral guilt, the war being just and lawful, and commanded by God. Thus God would preserve in their minds a dread and detestation of shedding blood. The spoil had been used by Midianites, and being now come into the possession of Israelites, it was fit that it should be purified.
Matthew Henry's Commentary on Numbers 31:23
Commentary on Numbers 31:19-24
(Read Numbers 31:19-24)
The Israelites had to purify themselves according to the law, and to abide without the camp seven days, though they had not contracted any moral guilt, the war being just and lawful, and commanded by God. Thus God would preserve in their minds a dread and detestation of shedding blood. The spoil had been used by Midianites, and being now come into the possession of Israelites, it was fit that it should be purified.