Researched by the Ignaria Editorial Team · Updated 2026-05-24
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 — God moving the soul toward righteousness, with the free will cooperating dispositively in the process. 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. One consequence of this divergence concerns assurance: Aquinas held that "man cannot judge with certainty that he has grace," while Luther and Calvin insisted that assurance rests precisely on Christ's finished work outside the believer, not on uncertain interior states. Watson later summarized the Reformation consensus in Trinitarian terms, assigning distinct roles to Father, Son, and Spirit without making any of them contingent on human cooperation.
Aquinas: Justification as Infusion of Grace
Aquinas presents justification as an act originating entirely in God's initiative, whereby divine grace is infused into the soul and moves the sinner toward righteousness. "The justification of the ungodly is brought about by God moving man to justice." The origin lies in God's act: "The entire justification of the ungodly consists as to its origin in the infusion of grace." Augustine's comparison underscores its significance — the justification of the ungodly surpasses even the creation of heaven and earth because the material order will pass away while this work endures eternally. Grace is thus not an external legal declaration but an interior principle that genuinely transforms the soul from its corrupted state.
Free Will and Human Cooperation
Aquinas maintains that God's movement respects the structure of rational human nature, which includes free will. "He moves man to justice according to the condition of his human nature. But it is man's proper nature to have free-will." Justifying grace is therefore not irresistible coercion but a divine initiative to which the human will cooperates dispositively — the habit of grace is "infused by God immediately without our operating as principal agents, but not without our co-operating dispositively by certain acts." Works proceeding from grace can merit eternal life condignly when flowing from the Holy Spirit's movement. Luther traced the opposite to a "wicked theory" — that doing one's best elicits grace from God — diagnosing it as a corruption that places confidence in human performance rather than divine mercy.
Luther's Forensic Alternative
Luther rejected the infusion framework entirely. Justification is not an interior transformation but a divine verdict — the sinner declared righteous on account of Christ's imputed merit. "By faith alone can we become righteous, for faith invests us with the sinlessness of Christ." The Formula of Concord defined the Reformed position: righteousness before God consists in "the gracious reconciliation or the forgiveness of sins, which is presented to us out of pure grace, for the sake of the only merit of the Mediator, Christ, and is received through faith alone." Faith functions instrumentally — it is "the means and instrument whereby we lay hold of Christ, and thus in Christ of that righteousness which avails before God." Even persistent sin in the regenerate does not undermine justification, because righteousness before God consists in Christ's imputed righteousness, not the believer's imperfect renewal.
The Crux: Certainty of Grace and Pastoral Consequence
One decisive consequence of the divergence concerns assurance. Aquinas held that "man cannot judge with certainty that he has grace" — because the principle of grace is God, who exceeds human comprehension, the believer cannot know with certainty whether justifying grace has been received. For the Reformation, this left the conscience in perpetual uncertainty before God's tribunal. Calvin diagnosed the scholastic tradition as having "deprived us of justification by faith, which lies at the root of all godliness." The Formula of Concord grounded assurance in objective imputation: justification "consists in the gracious imputation of the righteousness of Christ, without the addition of our works, so that our sins are forgiven us and covered, and are not imputed." When righteousness lies outside the believer in Christ's finished work rather than within uncertain interior states, assurance becomes possible.
What the primary sources show
"The justification of the ungodly is brought about by God moving man to justice... The entire justification of the ungodly consists as to its origin in the infusion of grace."
"God moves everything in its own manner... Hence He moves man to justice according to the condition of his human nature. But it is man's proper nature to have free-will."
"If, however, we speak of a meritorious work, inasmuch as it proceeds from the grace of the Holy Ghost moving us to life everlasting, it is meritorious of life everlasting condignly."
"Man cannot judge with certainty that he has grace, according to 1 Cor. 4:3,4: 'But neither do I judge my own self... but He that judgeth me is the Lord.'"
"By faith alone can we become righteous, for faith invests us with the sinlessness of Christ."
"The righteousness of faith before God consists alone in the gracious [gratuitous] reconciliation or the forgiveness of sins, which is presented to us out of pure grace, for the sake of the only merit of the Mediator, Christ, and is received through faith alone in the promise of the Gospel."
"But it is impossible to tolerate the impiety which, under the pretence of a twofold righteousness, undermines our assurance of salvation, and hurrying us into the clouds, tries to prevent us from embracing the gift of expiation in faith, and invoking God with quiet minds."