Midrash sa Oseas 2:21: Devarim Rabbah, Mekhilta DeRabbi Yishmael & Shemot Rabbah

Another matter, “that the Lord your God will maintain for you the covenant and the kindness” (Deuteronomy 7:12) – Rabbi Shimon ben Ḥalafta said: To what is the matter comparable? To a king who married a noblewoman. She brought two gems into the marriage, and the king, too, provided for her two corresponding gems. The noblewoman lost hers, and the king, too, took his. Sometime later, she arose and applied herself, and she [found and] brought back her two gems. The king, too, brought his. The king said: ‘These and those shall be crafted into a crown and placed on the noblewoman’s head.’ So, you find that Abraham gave his descendants two gems, as it is stated: “For I love him, that he will command his children and his household after him, [and they will observe the way of the Lord, to perform righteousness and justice]” (Genesis 18:19).¹⁶The qualities of righteousness and justice are compared here to two gems. The Holy One blessed be He, too, provided them with two corresponding gems, kindness and mercy, as it is stated: “The Lord your God will maintain for you the covenant and the kindness,” and it says: “He will give you mercy, and He will be merciful to you, and He will multiply you…” (Deuteronomy 13:18). Israel lost theirs: “For you have transformed justice into hemlock and the fruit of righteousness into wormwood” (Amos 6:12). The Holy One blessed be He took His, as it is stated: “For I have withdrawn My peace…kindness and mercy” (Jeremiah 16:5). Israel arose and applied themselves, and brought back those two gems. From where is this derived? From that which is written: “Zion will be redeemed with justice and its penitents with righteousness” (Isaiah 1:27). The Holy One blessed be He, too, brought His. From where is this derived? From that which is written: “For the mountains will move and the hills will collapse, [but My kindness will not move from you and the covenant of My peace will not collapse, said the One who has mercy on you, the Lord]” (Isaiah 54:10). Once Israel brings theirs and the Holy One blessed be He gives His, the Holy One blessed be He says: ‘These and those will be crafted into a crown and will be placed on Israel’s head,’ as it is stated: “I will betroth you to Me forever; I will betroth you to Me with righteousness, with justice, with kindness and with mercy. I will betroth you to Me with faithfulness and you will know the Lord” (Hosea 2:21–22).

And thus do you find that the exiles are gathered in, only in reward for faith, as it is written (Song of Songs 4:8) "With Me, from Levanon, My bride; With Me from Levanon shall you come (out of exile). You shall look (upon the redemption) merosh amanah" (homiletically: "in the merit of emunah [belief]). (Hoshea 2:22) "And I will betroth you to Me forever … and I will betroth you to Me with emunah."

Another interpretation: “This month shall be for you” – this is analogous to a king who betrothed a woman and wrote her [a marriage contract endowing her] minimally. When [the time] came to marry her, he wrote her [a contract endowing her with] numerous gifts, as a husband. So, this world is betrothal, as it is stated: “I will betroth you to Me forever” (Hosea 2:21), and He gave them only the moon, as it is stated: “This month shall be for you.” However, in the days of the Messiah, it will be marriage, as it is stated: “For your Husband is your Maker” (Isaiah 54:5). At that time, He will give them everything, as it is stated: “The wise will shine like the brightness of the sky; and those who lead the many to righteousness like the stars forever and ever” (Daniel 12:3).