Thomas Aquinas, the Angelic Doctor of the Roman Catholic church, is a Doctor of Theology located in the 13th century. When this is appreciated, things go better. If we could look at him, purely historically, this would be a better way. But instead, people, in particular, Protestant theologians are attempting to retrieve Aquinas’ theology, and the broader Thomist 
mantle in general, for what they see as a necessary corrective for the evangelical turn into heterodox and heretical positions in regard to doctrine and its subsequent praxis as it is applied to the daily lives of its adherents. But is Aquinas really the salve the evangelical Protestant churches need? Do we need to imbibe the theology of a Roman Catholic theologian with all of his thinking on nature/grace in tow? Just because certain Post Reformed orthodox theologians received Thomas’ theology, to varying degrees, in the ways they did, does this mean that evangelical Protestants ought to do this now?
My response to the above questions, of course, is a resounding and loud: No! Let me offer one reason, among many, why a Protestant Christian shouldn’t retrieve Thomas Aquinas’ theology in a positive way. Here we will hover, briefly, over Aquinas’ doctrine of grace, as that has been received in its Protestant Reformed orthodox dress (which, of course, is how many of the current Protestant theologians of retrieval are attempting to recover Aquinas’ broader lineaments for the churches). Richard Muller provides us with a good definition of this type of scholastic grace:
gratia: grace; in Greek, χάρις; the gracious or benevolent disposition of God toward sinful mankind and, therefore, the divine operation by which the sinful heart and mind are regenerated and the continuing divine power or operation that cleanses, strengthens, and sanctifies the regenerate. The Protestant scholastics distinguish five actus gratiae,or actualizations of grace. (1) Gratia praeveniens, or prevenient grace, is the grace of the Holy Spirit bestowed upon sinners in and through the Word; it must precede repentance. (2) Gratia praeparens is the preparing grace, according to which the Spirit instills in the repentant sinner a full knowledge of his inability and also his desire to accept the promises of the gospel. This is the stage of the life of the sinners that can be termed the praeparatio ad conversionem (q.v.) and that the Lutheran orthodox characterize as a time of terrores conscientiae (q.v.). Both this preparation for conversion and the terrors of conscience draw directly upon the second use of the law, the usus paedagogicus (see usus legis). (3) Gratia operans, or operating grace, is the effective grace of conversion, according to which the Spirit regenerates the will, illuminates the mind, and imparts faith. Operating grace is, therefore, the grace of justification insofar as it creates in man the means, or medium, faith, through which we are justified by grace…. (4) Gratia cooperans, or cooperating grace, is the continuing grace of the Spirit, also termed gratia inhabitans, indwelling grace, which cooperates with and reinforces the regenerate will and intellect in sanctification. Gratia cooperans is the ground of all works and, insofar as it is a new capacity in the believer for the good, it can be called the habitus gratiae, or disposition of grace. Finally, some of the scholastics make a distinction between gratia cooperans and (5) gratia conservans, or conserving, preserving grace, according to which the Spirit enables the believer to persevere in faith. This latter distinction arises most probably out of the distinction between sanctificatio (q.v.) and perseverantia (q.v.) in the scholastic ordo salutis (q.v.), or order of salvation.[1]
In the above definition it is gratia operans which serves instructive for our purposes. Here, we have Thomas’ doctrine of ‘created grace,’ which is, as he writes: “Strictly speaking, a supervening quality is not so much in existence itself, as a way in which something else exists; and so grace is not created, but men are created in it, established in a new existence out of nothing, without earning it: Created in Christ Jesus in good works.”[2] For Aquinas there is an interconnectedness occurring in reality. Thus, what he attempts to say is that, analogically, grace is not a created quality because Christ is still the first cause from whom this grace comes. The new existence is equivalent (i.e. for Thomas) to the accident, while the “out of nothing” is equal to the “exist” (i.e. or essence) of man. As such, in the final analysis, grace ends up being a created quality which is externally implanted into humanity. In other words, for Aquinas, grace is not an essential aspect of what it means to be human, and as such is thought of in terms of a created quality, albeit one that comes through Christ, and which becomes, in its reception, our gratia operans, or ‘operating grace,’ by which the elect cooperates with God in the attainment of their salvation. It is this type of thinking that gets modulated by the Federal theologians, and incorporated into their covenantal schemata of salvation, as this is functional in their Covenant of Works/Grace framework.
I offer this up as one example of a serious problem for those, especially of Baptistic orientation, who would attempt to recover the theology of Thomas Aquinas as if a salve for the evangelical churches. Aquinas is a Roman Catholic theologian, and as such, his total program, as that follows from his theology proper, will be corollary with Roman Catholic dogma and what they consider to be sacra doctrina. These Protestants who are recovering Aquinas as a boon, have seemingly strayed from their commitment to the historically Protestant ‘Scripture Principle.’ Their only response to that charge is to claim that Thomas’ theology is corollary with the teachings of Holy Scripture. But then this begs the question: in what way is Thomas’ teaching on a doctrine of grace, in our example, corollary with the teachings of Holy Scripture (particularly as Protestant’s read it)?
[1] Richard A. Muller, Dictionary of Latin and Greek Theological Terms: Drawn Principally from Protestant Scholastics Theology (Grand Rapids, Michigan: Baker Book House, 1985), 129-30. [emboldening mine]
[2] St. Thomas Aquinas, Summa Theologiae A Concise Translation, ed. Timothy McDermott (Westiminster: Christian Classics, 1989), 313.