One of the most significant Biblical promises is recorded in Genesis 49:10—“The scepter shall not depart from Judah, nor a lawgiver from between his feet until Shiloh comes (or, ‘until he comes to whom it belongs,’ cf. Ezek. 21:27); and to Him shall be the obedience of the people.” Further elaborated and confirmed by Samuel’s prophecy to David—“Your house and your kingdom shall be established forever before you. Your throne shall be established forever” (2 Sam. 7:16)—this is ultimately a promise of the coming Messiah, realized in Christ’s universal reign (1 Cor. 15:24–28), when “the Lion of the tribe of Judah” prevails (Rev. 5:5). But when Jacob uttered the prophecy, Judah may have realized with bitter irony that he had already disqualified his descendants from ever ruling over anyone.

According to Genesis 38:29 Judah fathered an illegitimate child named Perez, meaning “Breakthrough” (remember that name!) through his daughter-in-law, Tamar. Only many decades later did the awful consequences of Judah’s rash act became explicit in a law that seemed to cut off all hopes for Judah’s heirs to hold royal office: “One of illegitimate birth shall not enter the assembly of the LORD; even to the tenth generation none of his descendants shall enter the assembly of the LORD” (Deut. 23:2). To “enter the assembly” did not mean that illegitimates were prohibited from attending church, but that they were essentially denied citizenship: prohibited from holding office in the commonwealth of Israel. To violate God’s law in such a serious way, by corrupting one’s heritage, meant separation from the covenant in its official, legal aspects. But Judah’s descendants had been promised the royal line! What happens when God’s law and God’s promise conflict?

The answer is revealed only centuries later, and is the real reason for the existence of the Book of Ruth. To many readers, Ruth is a touching, yet sociologically odd, love story about a May-December romance between the young Moabite widow Ruth and the wealthy bachelor Boaz. Actually, it is much more. The real point of Ruth is to show the precise fulfillment of God’s ancient promise to Judah.

Remember the heartbreak of that promise: Judah’s descendants are the only ones eligible to hold royal office—yet because of Judah’s sin, they are legally prohibited from holding any office at all—until the tenth generation, a process of centuries. The Book of Ruth provides the crucial, indispensable link between God’s promise and its fulfillment. In fact, without the Book of Ruth, a crucial part of the history of redemption would be irretrievably lost: Ruth provides information about Christ’s genealogy that is unknown anywhere else.

The punch line of the book of Ruth—a narrative that looks almost irrelevant to those ignorant of Biblical legal history—recounts “the genealogy of Perez,” that illegitimate child of Judah, centuries in the past. Why should the ancient story of Perez, of all people, be important? Because, through no fault of his own, Perez happened to be illegitimate—yet in the royal line. By God’s infallible decree, Judah’s heirs were legally prevented, for hundreds of years, from holding any office at all—to say nothing of the kingship. Think for a moment: How long is a generation? No absolute definition is provided in Scripture—after all, each generation is different—but a good, round figure would be about 40–50 years. Conservatively speaking, God’s judgment on Judah’s sin prevented his descendants from claiming their rightful inheritance for about 400 hundred years. Think what this means in personal terms: If your ancestor in George Washington’s time had broken God’s law in such a serious respect, you would still be prevented from holding legal citizenship today! In the case of Judah’s descendants, God’s law and God’s promise were at odds, seemingly opposed—as irreconcilable as God’s mercy and His truth, as opposed as God’s righteousness and His peace. If God looks at you in terms of His absolute, inflexible righteousness and truth, can there be any room for mercy and peace?

Yet mercy and peace met together with righteousness and truth in the royal heir of Judah’s line: indeed, “Mercy and truth have met together; righteousness and peace have kissed each other” (Ps. 85:10). That is why the name Perez comes up as the people sing to Boaz: “May your house be like the house of Perez, whom Tamar bore to Judah, because of the offspring which the LORD will give you from this young woman” (Ruth 4:12).

Listen to the genealogy of Perez (Ruth 4:18–22): Perez (No. 1) begot Hezron (No. 2); Hezron begot Ram (No. 3), and Ram begot Amminadab (No. 4); Amminadab begot Nahshon (No. 5), and Nahshon begot Salmon (No. 6); Salmon begot Boaz (No. 7), and Boaz begot Obed (No. 8); Obed begot Jesse (No. 9), and Jesse begot David (No. 10). David was Judah’s tenth descendant. Providentially, the first descendant in Judah’s line to become King was also the first descendant who was legally eligible!

Ultimately, of course, the story of the Tenth Generation points to “the Son of God, Jesus Christ … For all the promises of God in Him are Yes, and in Him Amen, to the glory of God” (2 Cor. 1:19–20). As the angel Gabriel told Mary about her Son, Jesus, “He will be great, and will be called the Son of the Highest; and the Lord God will give Him the throne of His Father David. And He will reign over the house of Jacob forever, and of His kingdom there will be no end” (Luke 1:32–33).