What was the purpose of the Numbers 5 dusty(or "bitter") water test?
The Num. 5 "dusty water" oath/sacrifice is one of YHWH's good laws which is often misrepresented. It mercifully allowed a wife to clear her name from slanderous accusations. The husband (as stated in the law: Num. 5:13) has no witnesses to the alleged adultery. He is likely acting upon a rumor -- the sort of rumor which is easy to start, and devastating to a woman's reputation, if believed. Biblical law allowed the innocent woman to refute such a falsehood with no risk to herself, and for the cost of a small food sacrifice of barley (Num. 5:15). This sort of public ceremony would not be undertaken lightly by a husband. It would be a big risk to his reputation, if he wasn't pretty sure of her guilt.
The temple dust/ink/water concoction which the wife was required to drink would have been harmless, in itself. Ancient inks were typically made of lamp black (carbon) or iron oxide, and gum arabic. Basically harmless, in such small, dilute amounts.
This test, often called the "bitter water" test, has been compared to a trial by ordeal. It is the opposite of an ordeal, though, in the sense that the water mixed with dust was harmless in and of itself (and probably more healthy than the distilled, demineralized water which people drink on a regular basis nowadays), and required divine intervention to cause harm. Ordeals as used in man's law were typically actions that were harmful by nature (eg. drowning, fighting, touching boiling water), requiring divine intervention to guide the outcome or prevent the harm. Ordeals attempted to force God's hand in revealing a judicial outcome. In this sense, they are more akin to Gideon's use of the fleece (which is certainly not a pattern that we find encouraged in scripture).
We do find historical instances of Israelite kings seeking answers directly from YHWH, but this often seems to have been through the Urim and Thummim, which are scripturally obscure. There is also the case of the disciples casting lots for Judas' replacement (Acts 1:16-22). Again, the normativity of this method of decision-making is questionable. In any case, there was no legal issue (e.g. a person's life) at stake in the disciples' decision. Both of the candidates they were considering were fully-qualified and (from a human standpoint) equally good.
The fact that the dusty water test involves a food sacrifice might warn us to evaluate it as abolished with other Sinai sacrificial laws. Certainly, the involvement of an Aaronic priest and the sacrificial element do not seem to carry into the New Covenant. However, these features do not seem to be necessary to the ontology of this case.
What about the "dust of the temple"? Is there something special (holy, perhaps?) about the dust of the temple which makes the test especially effective? Possibly, but my intuition is that the dust was merely symbolic, and dependent upon a physical temple being present. For a people in which (nearly) every other part of their sacrificial system was tied to the temple/tabernacle, it would be surprising if we did not see this physical connection with the temple.
I submit that the general equity, "active ingredients" in the dusty water test are:
- the public, legal, self-maledictory oath of the wife (v. 22) -- inviting YHWH to act in judgment
- the willingness of YHWH to punish someone who sins in a high-handed way by making such a false oath
There is no reason to believe that #2 above is no longer true in the New Covenant age. So the two remaining questions are:
- Is a wife living in the New Covenant age allowed to make a legal oath/affirmation of her innocence?
- Can a wife be forced by her husband to make such an oath?
Again, I see no reason that we could deny #1 above. What about #2?
It isn't obvious to me that a wife could be forced to make such an oath even under the Sinai Covenant. An oath such as this is a form of witness testimony, and in no other legal case can someone be required to testify against themselves. It seems to me that a wife had the option simply to refuse the husband's request. This refusal would not have any civil, legal implications, but it might have been used by the husband as sufficient justification (along with other circumstantial evidence which caused him to suspect the unfaithfulness in the first place) to give his wife a writ of divorcement (Deut. 24:1). Nothing could prevent the husband from doing this (unless he himself had committed a prior sin that prevented divorce: Deut. 22:14-19), and if the marriage covenant had actually been broken by an act of unfaithfulness by the wife, then the husband's act of "putting her away" would not be a sin on his part (Matt. 19:9).