The contrast between Aquinas and Luther on justification is one of the most fundamental divides in Western theology. Aquinas understood justification as a real interior transformation through infused grace — the soul genuinely made righteous, requiring free will's movement toward God in cooperation with grace. Luther rejected this framework entirely: justification is forensic, a divine declaration that the sinner is righteous on account of Christ's imputed merit, with no prior works or interior transformation required for the verdict. Luther pressed this from Romans 4 — even Abraham's righteousness was credited by faith, not works, so that "no man can do any thing by works towards his becoming righteous." Calvin echoed this by insisting that pre-faith moral efforts are dead and produce no spiritual life, while Arminius later nuanced imputation by linking it to the Spirit's regenerative work without restoring merit.
"If therefore, Abraham be righteous by no works whatever, and if both he himself and all his works be left under sin, unless he be clothed with another righteousness, even with the righteousness of faith, it is quite manifest, that no man can do any thing by works towards his becoming righteous" — Luther's forensic conception of justification as imputed righteousness, rejecting any scholastic framework of infused grace or merit-based cooperation.
"Little children are saved only by faith, without any good works; therefore faith alone justifies" — Luther's clearest summary of sola fide as the governing principle of justification, used to demonstrate that righteousness is always credited by faith, never earned by prior moral achievement.
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