Paul’s questions in Rom 6 and following [“Are we to continue in sin so that grace may increase?” in 6.1; or “Shall we sin because we are not under law but under grace?” in 6.13; and others] must be read with the backdrop of chapters 1-5.
If someone is “made righteous” [this is how Catholicism defines justification] by faith in 1-5, then the expectation of an upright and holy person is that they will not sin. This is what righteous means. There was only one of those persons. But what if chapters 1-5 teach that when a person believes, he or she is “pronounced” and/or “declared” righteous? If this latter proposition be true, then it must also be pregnant with questions such as “Are we to continue in sin so that grace may abound?"
Further still, this pronouncement is such that it is not only a counting or a reckoning of one as righteous, but it is a declaration that ensures that one day the justified individual will be completely righteous like and with Jesus. Why else would Paul say that God predestined these to be “conformed to the image of His Son”? Why else would he continue his argument to say that all God predestined, God also called; and all God called, He also justified; and all God justified, He also glorified?
Paul would never link these near the close of his argument in chapters 5-8 and ask the kind of questions he did unless the declaration of righteousness in 1-4 secures the future reality of absolute righteousness forever in the new creation.