The water of bitterness was part of a "trial" used in ancient Israel when a husband suspected his wife of adultery but had no proof. This ceremony is described in Numbers 5:11–31. Some scholars think this belongs to a group of procedures called "trials by ordeal." In these trials, the accused person had to face some physical challenge to determine if they were innocent or guilty. People believed that a higher power who knew the truth would control the outcome.
In ancient Israel, like many old societies, women had few rights. An Israelite husband could use this test when he had no proof of his wife being unfaithful but simply had a “spirit of jealousy” about her (Numbers 5:11–14). Most likely, a pregnant wife would be tested if her husband thought the baby might not be his.
The suspicious husband would bring his wife to the priest with a special offering of rough barley meal (Numbers 5:15; compare Leviticus 2; 5:11). The priest would:
Bring the woman to stand "before the Lord"
Mix "holy water" (probably from the tabernacle water basin) with dirt from the tabernacle floor
Untie the woman's hair (possibly to show shame)
Place some of the barley meal offering in her hands (Numbers 5:16–18).
Then the priest made her take an oath stating that if she had been unfaithful, drinking the "water of bitterness" would bring a curse upon her. She agreed by saying "Amen, Amen"(verses 19–22). After writing the words of the curse, the priest would wash them off into the water. After making a ceremonial offering of a handful of barley meal, the priest made the woman drink the water (verses 23–26). If she was guilty, the result would be bitter suffering, causing the woman to become unable to have children (verse 27).