Paul, the now self-proclaimed slave of Christ Jesus, summarised his understanding of the gospel, the good news about Jesus, as follows:
Romans 1:16-17 For I am not ashamed of this Good News about Christ. It is the power of God at work, saving everyone who believes—the Jew first and also the Gentile. This Good News tells us how God makes us right in his sight. This is accomplished from start to finish by faith. As the Scriptures say: It is through faith that a righteous person has life.
Some translations show the end of verse 17 slightly differently, to say the righteous will live by faith. It’s shown as a quote from Habakkuk 2:4, which (in context) is:
Habakkuk 2:4-5 See, the enemy is puffed up;
his desires are not upright—
but the righteous person will live by his faithfulness [or faith]—
indeed, wine betrays him;
he is arrogant and never at rest.
Because he is as greedy as the grave
and like death is never satisfied,
he gathers to himself all the nations
and takes captive all the peoples.