It's Early
When I was doing campus ministry at Harvard, a student contacted me in distress, saying he was questioning his entire faith. So we met up at "ABP" or Au Bon Pain in Harvard Square. He told me that in his religion class, they'd learned that the best scholars agreed that the early Christians had slowly developed the idea that Jesus was God over decades. So he wanted to know, "If the first Christians didn't believe that Jesus was God, why should I?"
Paul likely starts his letter by talking about the gospel in language already familiar to the Christians in Rome. It can be harder to see in English, but Paul regularly builds these bridges in his letters. For instance, he says he's passing on what he received in 1 Corinthians 11:23-25 and 15:3-5. He mentions "trustworthy sayings" in 1 Timothy 1:15 and 2 Timothy 2:11-13. He's likely quoting hymns in Ephesians 5:14, Philippians 2:6-11, Colossians 1:15-20, and 1 Timothy 3:16. So, it wouldn't be surprising if Paul started by talking about the gospel with phrases the church in Rome already used. What Paul says about the gospel comes in two balanced phrases that can be removed without changing the grammar of his sentence. Try reading Romans 1:1-5 as if it read, "...concerning his Son, Jesus Christ our Lord, through him we have received grace and apostleship..." It's seamless. That's a hint that he's dropped a standard definition of the gospel into the middle of his sentence. Paul calls the same two claims "my gospel" in 2 Timothy 2:8. Both there and in Romans 1, the good news is that Jesus is descended from David and risen from the dead. In Romans, these claims are presented in a deliberate parallelism. The first claim is that God's Son was born of David's seed—according to the flesh. The second is that God's Son was appointed the Son of God in power—according to the Spirit of holiness. On their coronation day, Israel's kings were called God's son (see Psalm 2:7). So when Jesus rose from the dead, he rose all the way to a throne in heaven. The resurrection didn't make Jesus the Son, but revealed his power to us. Another clue that this is an early saying is that Paul prefers to write "the Holy Spirit" instead of "the Spirit of holiness." So, why does he use the phrase here? N.T. Wright argues that "the Spirit of holiness" is a Semitism. That is, it sounds like how a Hebrew way of talking gets written into Greek. So Paul and the Christians he wrote to already had a shorthand way of talking about Jesus as God's own Son, Israel's Messiah, and the risen Lord. When the church started in Jerusalem at Pentecost, there were "visitors from Rome (both Jews and converts)" who heard Peter explain that Jesus was descended from David, had risen from the dead, and was sending the Holy Spirit. We don't know who started the church in Rome, but Paul tells us in Romans 16:7 that two members of their church, Andronicus and Junia, were Christians before he was. So, we have good reason to think that, within a few years of Jesus' death and before any book in the New Testament was written, the first Christians were saying that Jesus, the Son of God, was also the human fulfillment of all the Messianic promises, and because he rose from the dead, he deserved our allegiance as Lord.
Why do you think the idea that Christians slowly invented the belief "Jesus was God" spreads so easily?
What does it tell us about Jesus that he is the human descendant of David and the Son of God?
When you have doubts, who would you call? (If you don't have someone, you can always ask a question in our online community at up.uncommonpursuit.net).
Together with a friend, pray the creed that the early Christians in Rome likely had: "Jesus Christ our Lord, descended from David according to the flesh, appointed to be the powerful Son of God according to the Spirit of holiness by the resurrection of the dead, thank you for being good news for us. Amen."