Numbers 5:11-31 contains one of the weirdest rituals you will find in the Old Testament. It is preceded by a law dealing with purity in Israel: everything impure, even if it looks impure, is to be expelled from Israel’s camp.
In our own day we find this more than harsh. Is it not our calling to minister to those who are ill and suffering, no matter how repugnant might be their condition? We wouldn’t think of expelling them from our presence.
God, however, is sending a message for all time: He regards impurity of any kind to be unworthy of His presence. It was an object lesson to Israel.
It is in the context of this matter of purity that God takes up the case of the woman who has been unfaithful to her husband. Note that this is different from Leviticus 19 in that it deals with suspicion rather than evidence. The seriousness of the matter is underscored with the elaborate ritual associated with it (it’s the only ritual in the Old Testament where the precise words of the ritual are recorded). In all cultures, ritual conveys importance.
Would dust from the floor of the tabernacle, mixed with holy water in a clay dish, really cause physical illness and barrenness?
But God could cause both and He, knowing the hearts of all, would be called to act. The whole end result was, in fact, in the hands of God.
So why the ritual?
To convey the seriousness of the charge, and the seriousness of the sin.