Catholics are often surprised to learn that there are Evangelical Protestants who claim to be Thomists. When I was a Protestant, I was one of them. What attracts these Evangelicals are Thomas’s views on faith and reason, his philosophy of the human person, command of Scripture, and intellectual rigor. Some of them think that on justification, Thomas is closer to the Protestant Reformers than to the Catholic view (as taught in the 1994 Catechism of the Catholic Church ). The late Presbyterian theologian John Gerstner, for instance, claimed that with St. Augustine, St. Thomas “taught the biblical doctrine of justification so that if the Roman Church had followed Aquinas the Reformation would not have been absolutely necessary.” Others have made similar arguments, but they are spectacularly wrong. As usual, it all hinges on understanding faith and works.
For St. Thomas , justification refers not only to entrance into the family of God at Baptism – administered for the remission of sins – but to the infusion of sanctifying grace at Baptism and all the subsequent graces that work to transform the Christian from the inside out. Consider, for instance , Aquinas’s explanation of sanctifying grace as habitual grace: “a double effect of grace, even as of every other form; the first of which is ‘being,’ and the second, ‘operation.’” For example, “the work of heat is to make its subject hot, and to give heat outwardly. And thus habitual grace, inasmuch as it heals and justifies the soul , or makes it pleasing to God, is called operating grace; but inasmuch as it is the principle of meritorious works, which spring from the free-will, it is called cooperating grace.”
Because God is the sole mover in the infusion of habitual grace, it is entirely attributable to Him. In technical language, this is called operating grace. But if habitual grace is supposed to heal and justify the soul, and the soul has by nature certain powers to think and act, then this healing and justification must manifest itself in the activities of the soul. Thus, these acts allow us to cooperate with God for our inward transformation. Aquinas calls this cooperating grace, since any meritorious acts performed by a soul infused with habitual grace by God would lack merit without that grace and thus without God’s cooperation. Writes St. Thomas : “God does not justify us without ourselves, because whilst we are being justified we consent to God’s justification [justitiae] by a movement of our free-will. Nevertheless this movement is not the cause of grace, but the effect; hence the whole operation pertains to grace.”
St. Thomas Dedicating His Works to Christ (Santi di Tito)
For Aquinas, justification and sanctification are not different events, one extrinsic and the other intrinsic, as the Protestant Augusburg (1530)  and Westminster (1646)  confessions teach. Rather, “sanctification” is the ongoing intrinsic work of justifying, or making the Christian rightly-ordered by means of God’s grace, the same grace that intrinsically changed the believer at the moment of her initial “justification” (i.e., at Baptism) into an adopted child of the Father . Writes Aquinas , “Augustine says (De Gratia et Lib. Arbit. xvii): ‘God by cooperating with us, perfects what He began by operating in us, since He who perfects by cooperation with such as are willing, begins by operating that they may will.’ But the operations of God whereby He moves us to good pertain to grace. Therefore grace is fittingly divided into operating and cooperating.” For Aquinas, justification is as much about getting heaven into us as it is about getting us into heaven.
The 1994 Catechism  is of a piece with St. Thomas: “The first work of the grace of the Holy Spirit is conversion, effecting justification in accordance with Jesus’ proclamation at the beginning of the Gospel: ‘Repent, for the kingdom of heaven is at hand.’[Matthew 4:17]. Moved by grace, man turns toward God and away from sin, thus accepting forgiveness and righteousness from on high.” And it uses the language of cooperating grace in its account of human merit and the role it plays in justification: “The merit of man before God in the Christian life arises from the fact that God has freely chosen to associate man with the work of his grace. The fatherly action of God is first on his own initiative, and then follows man’s free acting through his collaboration, so that the merit of good works is to be attributed in the first place to the grace of God, then to the faithful. Man’s merit, moreover, itself is due to God, for his good actions proceed in Christ, from the predispositions and assistance given by the Holy Spirit.”
Oddly, several Evangelicals cite part of this as evidence that the Church teaches “grace plus works,” even though that’s not what the Catechism is saying. The Catechism teaches, “The merits of our good works are gifts of the divine goodness.” (Does that sound to you like “grace plus good works”?) Consequently, one’s cooperation does not take away from the fact that justification is a work of God, just as Christ’s human nature does not diminish his divine nature, and just as the Bible being authored by human beings is not inconsistent with it being God’s Word. Thus, St. Thomas was more a proto-Tridentine Catholic than a proto-Protestant.
What then accounts for this misreading of the Angelic Doctor? Love. No serious Christian can read St. Thomas without being impressed by his intellect and philosophical acumen, but also his encyclopedic knowledge of Scripture. This no doubt has enkindled in even in the coolest of Protestant hearts a warm affection for Aquinas. These smitten scholars unconsciously find creative ways to make it seem as if a thirteenth-century Dominican Friar was a lonely beacon in a papist fog destined to be vindicated by a sixteenth-century Augustinian Monk. Unfortunately, or fortunately, it is not so.