In Romans 1:17, Paul quotes: 'The righteous will live by ___.'

Answer: Faith

The gospel reveals the righteousness of God that comes by faith, as it is written: 'The righteous will live by faith.'

“For the gospel reveals the righteousness of God that comes by faith from start to finish, just as it is written: “The righteous will live by faith.””— Romans 1:17 (BSB)