According to Hebrew thought, when people took an oath they called down a curse upon themselves if they were not telling the truth (Mark 14:71) or if, after making a promise, they did not keep their word (2 Sam 3:8-10). In swearing by the name of God, they were inviting God to take decisive action against them should they be false to their oath (1 Sam 19:6; 2 Kings 2:2; Jer 42:5; Ezek 17:18-19; see CURSE). There were various rituals that people followed in swearing oaths. Where two parties bound themselves to a contract by oath, they sometimes carried out a ritual where they passed between portions of slaughtered animals, calling down the fate of the animals on themselves should they break their oath (Jer 34:18; cf. Gen 15:9-20).
A person might, in swearing an oath, raise one hand above the head or, if swearing to another, place one hand under the other person’s thigh (Gen 24:2-3; Deut 32:40). People could swear oaths before local judges or at the sanctuary altar (Exod 22:10-11; 1 Kings 8:31). A special ritual was available when a woman was suspected of adultery and she wanted to swear her innocence (Num 5:11-31). When Israelites swore by the name of God, they were to be careful not to swear falsely (Lev 19:12).
Under no conditions were they to swear by the name of a false god (Amos 8:14). If they swore a rash oath and later regretted it, they could ask forgiveness through presenting a guilt offering and making any compensation that may have been necessary (Lev 5:4-6; 6:5; cf. 1 Sam 14:24-29). Even God sometimes bound himself by an oath; for example, in his covenant promises to Abraham (Gen 15:5-20; 22:16-17; Luke 1:68-73; Heb 6:13-14), to David (Ps 89:34-36; Acts 2:30), to the messianic king (Ps 110:4; Heb 7:15-22,28), and to his redeemed people (Heb 6:16-17). Although he had no need to take an oath (since his word is always sure), in his grace he confirmed his promise by an oath, so that believers might be doubly certain of their ultimate salvation (Heb 6:17-20). Wrong practices developed among the Jews concerning the taking of oaths. Some considered that if, in swearing an oath, they did not actually use the name of God, they were not bound by that oath. They felt no guilt if they swore ‘by heaven’, ‘by earth’, ‘by Jerusalem’ or ‘by the head’ and then broke their promise, for such oaths did not use God’s name. Jesus told them that if they were truthful and honest in all their day-to-day behaviour, they would not feel the need to swear oaths at all. Everything a person says should be true and straightforward (Matt 5:33-37; 23:16; James 5:12).