The greatest cleansing is the spiritual washing that washes away the filth of the soul. The inspired word speaks of such a washing: “The Lord shall wash away the filth of the sons and daughters of Israel and shall wash away the blood from their midst.” This refers to the blood of immortality as well as the killing of the prophets. He means by this purification, seen from the added phrase, “by the spirit of judgment and by the spirit of burning.” The washing of the body, however, is physical and is accomplished only by water. In fact, it can even be done in fields far away from the baths.
Source: The Instructor Book 3