The actus fidei in Protestant Reformed theology was an attempt to detail the component parts of what makes up ‘saving faith’ as it were for the recipients of salvation in Christ. What we will cover in this post follows along the lines we touched upon in the last post when we took a look at an ontology of grace. What should stand out to you as you read this is the elevation that intellect and will attain in this schema and explication of ‘faith’. It comes back to an issue of anthropology, of a Thomist intellectualist type (but we will have to cover that more later).
actus fidei: the act, actualization, perfecting operation, or actualizing operation of faith; in addition to their objective, doctrinal definitions of fides (q.v.), the Protestant orthodox also consider faith as it occurs or is actualized in the human, believing subject. In the subject, faith can be considered either as the disposition or capacity of the subject to have faith (habitus fidei, q.v.), which in case of saving faith (fides salvifica) is a gracious gift of God, or as the actus fidei, the act or actualizing operation of faith, in which the intellect and will appropriate the object of faith (obiectum fidei, q.v.). The actus fidei, then, can be described by the Lutheran and Reformed scholastics as an actus intellectus and an actus voluntatis, an operation of intellect and of will. Both notitia (knowledge) and assensus (assent to knowledge) belong to intellect, while the apprehensio fiducialis, or faithful apprehension, of that knowledge is an act of will. Saving faith in Christ comprises, therefore, the actus credenda in intellectu, the actualization of believing in the operation of the intellect, and the actus fiduciae (q.v.), or actus fiducialis voluntatis, the actualization of faithfulness in the operation of the will. The soul may be considered as the subiectum quo (q.v.), or “subject by which,” of faith, since soul may be distinguished into the faculties of intellect and will.
The scholastic language of faith as actus must not be construed as a description of faith as an activity that accomplishes, for the mind and the will, a saving knowledge of and trust in Christ. Such a view would constitute a denial of the doctrine of justification by grace alone (see iustificatio). Instead, the language of habitus fidei and actus fidei, of the disposition or capacity for faith and the actuality or perfecting operation of faith, needs to be understood in the context of the scholastic language of potency (potentia) and act, or actuality (actus). The disposition, or habitus, is a potency for faith that can be actualized as faith. The act or actus of faith, although it may be defined as an operation, is not an activity in the sense of a deed or a work, but an operation in the sense of an actualization in which faith comes to be faith or, in other words, moves from potency to actuality.
The Reformed orthodox further distinguish the actus fidei into several parts. The first distinction is twofold: an actus directus and an actus reflexus. The actus directus fidei, or direct operation of faith, is faith receiving or, more precisely, having its object. By the actus directus fidei an individual believes the promises of the gospel. The actus reflexus fidei, the reflex or reflective operation of faith, is the inward appropriation of the object according to which the individual knows that he believes. These two acts can be further distinguished since, in particular, both notitia and assensus can be considered as actus directus. The actus directus can be distinguished into (1) an actus notitiae, or actualization of knowledge, and (2) a twofold actus assensus, or actualization of assent (assensus theoreticus and assensus practicus), consisting in an actus refugii, or actualization of refuge, and an actus receptionis et unionis, an actualization of reception and union. By way of explanation, each of these components of the actus fidei is direct insofar as it refers to the object of faith as appropriated. This is clear in the case of the actus notitiae according to which the obiectum fidei, the supernaturally revealed Word of God, belongs to the intellect, and also in the case of theoretical assent according to which the intellect agrees to the certainty of the truth of its knowledge. The assensus practicus et fiducialis, or practical and faithful assent, still belongs to the intellect, which here recognizes as certain and as the obiectum fidei, not only scriptural revelation, but that revelation of grace and sufficient salvation in Christ which God has promised to believers. The actualization of refuge follows immediately as the realization that Christ himself and union with him provide faith with the means of salvation. This actus is primarily of the will but still direct. Finally, on the ground of all that has preceded, but also now as a result of the actus voluntatis, or actualization of will toward Christ, there is an actus receptionis sive adhaesionis et unionis Christi, an operation of the reception of, adhesion to, and union with Christ. The next operation of faith is the actus reflexus in which the soul reflects upon itself and knows that it believes what it believes and that Christ died for it. Whereas the actus reflexus is primarily an actus intellectus, the final actus fidei belongs to the will. The actus consolationis et confidentiae, or actuality of consolation and confidence, is an acquiescence of the will to Christ and the knowledge of salvation in Christ. The scholastic analysis of the actus fidei is, in short, an attempt to isolate and define the elements of faith which must all be actualized in the believer if the graciously given disposition toward faith, the habitus fidei, is to bear fruit in a full realization of fides.
Stephen Strehle in his 1996 book was right to entitle it The Catholic Roots of the Protestant Gospel: Encounter Between the Middle Ages and the Reformation, because as Muller’s definition of saving faith within Protestant Reformed orthodox theology illustrates how that is so. What we have is a Protestant reification of medieval and Catholic (Thomist, largely) Aristotelian language when it comes to describing faith and salvation. Even within Muller’s definition we see how he qualifies a shift that took place among the Protestant deployment of the ‘Catholic’ language and indeed the Protestant usage of it; particularly when we see the language of habitus and actus pop up regularly.
Habitus and actus are both fundamental parts of Aristotle’s philosophy and anthropology of virtue and Thomas Aquinas’ theological appropriation of that grammar within his medieval context. What is also present, and highly Aristotelian, about this definition of actus fidei is its constant appeal to a faculty psychology i.e. tripartite faculty psychology wherein mind, will, and affections are understood to be the constitutive parts of what it means to be human. Of course in the scholastic schema what we are going to get an emphasis upon, just as we do in Thomas Aquinas’ Roman Catholic theology is an emphasis upon the intellect/will; since both the Protestant Reformed orthodox and Roman Catholics, by and large, hold to what is called a Thomist intellectualist anthropology wherein the intellect/will are understood to be the defining components of what it means to be human. This is significant, particularly when we start thinking about a ‘biblical’ and genuinely Christian spirituality juxtaposed, indeed, with the Bible’s emphases—which focuses much more on the heart rather than the intellect (if we even want to appeal to a tripartite faculty psychology in the first place).
On a more negative note: it is hard for me to understand, after engaging with Protestant Reformed orthodox theology how those who adhere to it in repristinating form can maintain that what they offer just is “biblical,” and then hear them critique someone like Karl Barth or Thomas Torrance as if their theologizing is outside the bounds of Christian orthodoxy. It is absolutely ad hoc and petitio principii to make such accusations; all Christians, of whatever stripe, do theological exegesis and maneuvering in their attempt to lay bare what is there in the text of Holy Scripture and its attestation to its reality in the life of God in Jesus Christ.
I would simply ask adherents to what is ostensibly the orthodox faith of Reformed Christians, to at least be humble enough to admit that their positions are just as “theological” and less stridently or prima facie “biblical” as those they believe are outside the bounds. The standard is Holy Scripture and its regulative reality in Jesus Christ; intramural critiques done either way ought to stay at the level of material theological engagement, and labels bandied around such as “heretic” or even “heterodox” ought to be done away with (unless of course we actually do encounter heresy or heterodoxy).
 Richard A. Muller, Dictionary of Latin and Greek Theology Terms: Drawn Principally from Protestant Scholastic Theology (Grand Rapids, Michigan: Baker Book House, 1985), 21-23.