Exodus 28 describes the special garments that would be made for the Israelite high priest "for glory and for beauty" (verse 2). These garments were designed, in particular, to remind the high priest of his weighty responsibilities to both represent the people of Israel and appear before the Creator of the Universe.
The garments were made from the same fine fabrics as the curtains of the tabernacle---"gold, blue and purple and scarlet yarns, and fine twined linen" (verse 5). Since the yarns were made of wool, these were clothes including a mixture of wool and linen. This fact helps explain the general prohibition of mixed fabrics in Israel (Lev 19:19; Dt 22:11). Such mixtures apparently were reserved for tabernacle use.
The priestly vestments included a ceremonial vest, the ephod, to which would be attached a special breastpiece. The breastpiece consisted of a nine inch by nine inch square of fabric, doubled over to form a pouch (vv 15-16). On the front of the breastpiece were twelve precious gems, arranged in four rows of three gems each. The gems were engraved with the names of the twelve tribes of Israel. When the priest served at the tabernacle wearing the ephod, he thus bore the names of the twelve tribes "on his heart" (verse 29).
The breastpiece was called the "breastpiece of judgment," reflecting the fact that it was used in seeking God's judgment on matters of national import. Beginning with Joshua, when a leader of Israel sought God's will in an important matter, he would go to the high priest to make a formal request for guidance (Num 27:18-21). God somehow would communicate that guidance through the breastpiece.
Exodus 28:30 explained that the "Urim and Thummim" would be placed in the breastpiece and presumably would be involved in the reception of revelation from God. The phrase "Urim and Thummim" means "lights and perfection," or perhaps "perfect light." One explanation is that through the breastpiece, God would illuminate the path of the nation and its leaders, steering them in the right direction.
The Bible does not tell us specifically how God's will was communicated through the breastpiece. One proposal is the the Urim and Thummim were two stones that could be used to ask God yes or no questions. Guided by God, the high priest would pull out one of the stones from the pouch to reveal God's answer.
This idea seems to find support from 1 Sam 14:36-41. During a military campaign, Saul proposed making a night attack against the Philistines. The priest suggested he seek God's guidance, and Saul asked, "Shall I go down after the Philistines? Will you give them into the hand of Israel?" But God did not give an answer.
Concluding that God's refusal to answer was due to a sin on Israel's part, Saul asked a follow-up question. Had the sin been committed by Saul and/or his son Jonathan? Or had it been committed by some other Israelite?
"O LORD God of Israel, why have you not answered your servant this day," Saul asked. "If this guilt is in me or in Jonathan my son, O LORD, God of Israel, give Urim. But, if this guilt is in your people Israel, give Thummim" (verse 41).
This translation from the ESV, based on the Greek Septuagint text, suggests that the Urim and Thummim represented the two possible answers to a yes or no question. It should be pointed out, however, that the Urim is not mentioned in the Masoretic Text.
The Bible mentions a number of instances where God's counsel was sought through a formal inquiry, Some of the questions were simple yes or no questions, but some were more complicated, with answers from God that involved more than a simple affirmative or negative response.
Doug Bookman, a Christian scholar, conducted a detailed study of the Urim and Thummim in a 2001 Dallas Seminary ThD dissertation. He concluded that the detailed nature of some of the questions and answers imply that God's communication with the high priest involved more than a simple binary lot.
Bookman's findings are in agreement with Jewish tradition. Rabbinic traditions about the Urim and Thummim envision the high priest placing in the pouch a piece of parchment upon which was written the Divine Name. God communicated his answers to formal inquiries by causing the gems on the breastpiece, and the letters inscribed in the gems, to be illuminated.
For example, Israel after the death of Joshua asked God, "Who shall go up for us against the Canaanites, to fight against them?" (Judges 1:1). God answered, "Judah shall go up; behold, I have given the land into his hand" (verse 2). Perhaps God caused the gem representing the tribe of Judah to light up and then used the letters inscribed in the gems to send the rest of this message.
We do not know how often, or for how long, this method of seeking God's will was used. The last time the Urim and Thummim is mentioned in the Bible is in Ezra 2:63; Neh. 7:65. There were some returning from exile who could not document their status as priests, and so their status was left up in the air until the Urim and Thummim could be used. At that point people were aware of the Urim and Thummim, but it's not clear if there was anyone left who could use it.
Bookman proposes that the Urim and Thummim was underused. To take advantage of tbe Urim and Thummim, the leadership needed to be submitted to God and sincerely seeking his will. It may have too often been the case that Israel's leadership knew it was doing wrong and didn't want to be corrected.
