Ruth is the story of a little foreign girl who came out of paganism and idolatry in the land of Moab. She came from a people who were in many senses an outcast people, and she came into a knowledge of the Lord God of Israel, as Boaz said, “Under whose wings thou art come to trust” (Ruth 2:12).
Ruth has only four brief chapters, but it is a mighty midget with a mighty message. In fact, it has several messages. It gives a genealogy that leads to the Lord Jesus Christ, and it explains His coming from the line of David. There are commentators who take the position that the primary purpose of the Book of Ruth is to give the genealogy. While I agree that this is an important purpose of the book, I do not believe it is the primary purpose. Keil and Delitzsch make this statement: “The last words of verse 17, ‘he is the father of Jesse, the father of David,’ show the object which the author had in view in writing down these events, or composing the book itself. This conjecture is raised into a certainty by the genealogy which follows, and with which the book closes.” The Book of Ruth is very important in connection with the coming of Jesus Christ into this world. Without this little book, we could not connect the house of David with the tribe of Judah. It is an important link in the chain of Scripture that begins with Genesis and goes right down to that stable in Bethlehem and to the Cross, to the crown, and to the throne of David on which our Lord will someday be seated. This is a very definite reason why Ruth is included in the canon of Scripture.
However, the primary purpose of the Book of Ruth is the presentation of an important phase in the doctrine of redemption. Redemption is possible only through a Kinsman–Redeemer. God could not redeem apart from a Mediator. Since only God could redeem, it was necessary for Him to become that person. Boaz furnishes the only figure for the Kinsman–Redeemer aspect of redemption which is so essential for any proper theory of the Atonement. This little Book of Ruth comes down to our level and tells the commonplace story of a couple who love each other. They were ordinary folk, average folk, and their love story is a mirror in which we can see the divine love of a Savior for you and me. As we proceed into the Book of Ruth, we see this wonderful love story unfold before us.
(McGee, J. Vernon. Thru the Bible Commentary, Vol. 11: Ruth. Nashville, TN: Thomas Nelson Publishers, 1991.)