"For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek.What is the saving power of God? It is the gospel. What is the gospel? It is the saving power of God. How does this power save us? By believing in Jesus Christ. What is the content of the gospel? The disclosure of the righteousness (or, justice) of God. What is the righteousness/justice of God? (And, to go back a question or two, what does it mean to believe in Jesus Christ?)
For in it the righteousness of God is revealed from faith for faith, as it is written, 'The righteous shall live by faith'."
Ah! Read Romans 1:18 to the end ...
Incidentally, Paul restates all this at the end, Romans 16:25-27.