The passage describes an unusual way to determine God’s will.
Quite often in the Bible yes/no questions were answered by the casting of lots, or, as here, by consulting the Urim and Thummim, probably differently colored stones. The first king of Israel, here, unravels a bit of the A mystery is something secret, hidden and not perceived by ordinary means. In the book of Daniel a significant mystery is revealed through divine revelation (Daniel 2); Paul speaks of a mystery of God in Romans 11 and again in Ephesians 3. In speaking of... that surrounds these cultic objects. Should the Urim stone appear (however that would be!), it would indicate guilt on Saul’s part as well as Jonathan’s. Should the Thummim appear, however, the guilt of the people of Israel would be indicated. While it sounds somewhat “dicey” to us, Israel was convinced that, though “the lot is cast into the lap,” “the decision is the LORD’s alone” (Proverbs 16:33).