This depiction of reason proceeds with the similitude of washing or filtration: the offering was intended to decontaminate . . . a people for himself. Here the symbolism isn't of absolution (look at 3:5; Eph 5:25-26). Or maybe, as the Old Testament setting of the reference recommends (Ezek 37:23), "washing" means God's demonstration of decontaminating or blessing his wayward individuals from the contamination of excessive admiration—asserting a people out of the wicked world.