Some high theology that devolves into trying to make sense of the phrase, “Repent, and believe the good news.”

Psalm 25:1-10

The entirety of Psalm 25 is cited in Formula of Concord, Solid Declaration, Article 2: Free Will (FC, SD 2.81) as a reference to the works of Augustine and his reflection on this psalm through Paul's writings to clarify that when someone comes to faith, while they are reborn, they are not a new creature in the sense that the previous person is entirely destroyed. We do become new people through the daily dying and rising, but this work of the Spirit in our lives is not through removing who we are now from existence. The idea is that we are made new, not that new "we"s are made.

1 Peter 3:18-22

For Christ also suffered for sins once for all, the righteous for the unrighteous, in order to bring you to God. He was put to death in the flesh, but made alive in the spirit…
— 1 Peter 3:18

Verse 18 is cited in Formula of Concord, Solid Declaration, Article 8: Person of Christ (FC, SD 8.37) as an example of why to be cautious when talking about the distinction between Jesus' human and divine natures. When thinking about how God works in Jesus, we need to keep two thoughts in mind: that the person of Jesus is both fully human and fully divine; and that we should not mix what is possible through Jesus' divine nature with what Jesus does through his human nature. After this citation (FC, SD 8.39-43), large sections of Luther writing against Zwingli are quoted. If you want to run this thought down, I would recommend starting with Luther's Works, American edition, volume 37 starting at page 209.

Mark 1:9-15

…and saying, “The time is fulfilled, and the kingdom of God has come near; repent, and believe in the good news.”
— Mark 1:15

Verse 15 is cited six times throughout The Book of Concord. You'll notice a theme with these citation because the point is not subtle, “Repent, and believe in the gospel.”

The first quote is in the Apology of the Augsburg Confession, Article 12: Repentance (AP 12.45), which Melanchthon uses to note the full idea of repentance. His argument is that first comes contrition—grieving your sinfulness and your sins—and then comes belief—rejoicing in the good news. Jesus provides this in this simple phrase. Step 1: Repent. Step 2: Believe in the gospel.

Verse 15 is next quoted in the Smalcald Articles, Part 3, Section 3: Repentance (SA 3.3.4) to show how God guides people once they’ve experienced the judgment of the law. “Become and act otherwise, and believe my promise” (SA 3.3.5).

The next two citations of verse 15 are in the paired articles of the Formula of Concord. In the Epitome, verse 15 is cited in Article 5: Law and Gospel, Affirmative Thesis 5 (FC, Ep 5.6), to show an instance of when the word “gospel” is used to mean the general proclamation of God’s law and God’s promise. This matters because the word “gospel” is not always used that way. It is also used in reference to the specific declaration of God’s promise. For those who follow Luther’s ideas, this becomes an important ambiguity to which attention must be given.

This difference is more fully unpacked in the Solid Declaration, Article 5: Law and Gospel, where verse 15 is cited to make the opposite point, that there is a strict proclamation of the gospel, which is just announcing God’s grace (FC, SD 5.6). This is explained a few lines later (FC, SD 5.8) as an example of the contrast between repentance and faith. When held separately, repentance is feeling sorry for your sins, but that is not the same as faith, which is believing that God, in Christ, forgives us.

The final citation if verse 15 is in Formula of Concord, Solid Declaration, Article 11: Election (FC, SD 11.67) showing two clear ideas. First, that election is only through Jesus, which we know because, second, Jesus announces God’s will—that all people should repent and believe. God calls those of us who would claim to follow Jesus to proclaim this news to all people so that everyone might learn to trust God.

TheoThru

God wants you, broken and fractured you, and to prove it became one of us. Jesus makes known what God wants and how God will make this possible. Repent and believe. Let the law judge you, and trust in God’s forgiveness because of Jesus. God loves us as we are because God loves who we will become.