If an Israelite man suspected his wife of adultery, then he had the right to have her "tested" by the priests. Numbers 5:11-31 describes this process – yet another ritualistic law that causes most readers to shrug their shoulders in confusion and skip on to the next section.
The NKJV refers to this section of scripture as “Concerning Unfaithful Wives.” A man could bring his wife before the priest any time that he thought she might have been unfaithful to their marriage. This could be done as often as the husband chose, and there was no analogous procedure for women to test their husbands. The priest would take some water and add to it a pinch of dust from the floor of the tabernacle. He would also etch an oath into a clay tablet and then scrape the words off into the water. The woman would swear the oath, which said that she had not committed adultery and that she would be cursed if she had, and then she would drink the “bitter water the brings a curse.” Afterwards, if she had been lying, she would become ill: it says that her stomach would swell and her thigh would rot. While the punishment may seem a bit vague, it was probably some condition associated with barrenness, since, if she were telling the truth, it says that she would be free and allowed to have children.
To understand the purpose of this law, we should consider the symbolism of the biblical institution of marriage. God ordained marriage to be between a man and a woman, and for the woman to be subservient to the man “as to the Lord” (Ephesians 5:22). In numerous places throughout the bible, the future relationship of God’s people with Christ is pictured as a marriage, with Christ being the groom and the Church being the bride (Revelation 19:7-9, for example). Human marriage is a “shadow of the things to come” (Hebrews 10:1) - it was created in order to teach us about the relationship that we are supposed to have with God. Likewise, the focus of this particular law was not to help Israelite men catch their adulterous wives – it was pointing to our relationship with (and eventual marriage to) God!
Once we understand this key, the interpretation is simple: God, our future Husband, reserves the right to test us. We have to drink the bitter water of His Word, which comes with a choice between life and death – blessing and cursing. If we have committed spiritual adultery through any form of idolatry, then we will be cursed and cut off from Him. On the other hand, if we are faithful to Him, then we will be blessed and found suitable for marriage. Furthermore, just as in the physical law, our Husband may test us as often as He chooses, and we have no right to test Him in return: He is faithful in all things.