Catholic theology teaches that Adam was in some kind of “state of grace” before the fall. This grace was some kind of “stuff” that God applied that made Adam really special.
The notion of infused grace is part of a wider ontology that is applied to the original created state. Augustine had maintained that Adam was upheld in righteousness by an enabling grace that was added to an ontologically unstable nature. The fall occurred with the withdrawal of this donum superadditum and the consequent shift of vision from the invisible and intellectual to the visible and corporeal. Following Augustine, the early medieval scholastics distinguished between an operative grace (liberating the will from its bondage) that always precedes human effort and a cooperative grace that assists human effort. Following Aquinas, the Council of Trent decreed that through prevenient grace God prepares the soul, while it is “through his stimulating and assisting grace [that individuals] are disposed to convert themselves to their own justification” (from Trent session vi, Chapter 5) -- [Horton's original erroneously attributes "canon" 5]. It is worth noting Wilhelm Pauck’s observation that the verb ekkechutai in Romans 5:5 (“the love of God is shed abroad in our hearts by the Holy Spirit” – from the KJV) was rendered diffusa est in (was diffused) by the Vulgate, and this became a key basis for the doctrine of infused habits. (Horton pg. 191)
(Incidentally, this is another mistranslation in the Vulgate that got written into Catholic Dogma. This one, however, had some effect on history).
Horton doesn’t satisfy himself with this particular citation, however. He traces this very thoroughly, along with the consequences of this doctrine.
In Horton's Covenant theology, man, created God's Image, has all of the "ontology" he needs. The sin was a legal breakage of the covenant – a “forensic” fall, as it were:
According to federal (covenant) theologians, Adam and Eve were never in a state of grace before the fall. Endowed in their creation with all of the requisite gifts necessary for fulfilling God’s eschatological purposes, there was nothing lacking requiring a gracious supplement.
This reveals a fundamentally different understanding not only of the original condition of humanity in Adam (under a covenant of works), but of grace itself. After all, “the image of God is not a superadded gift but integral to the essence of humanity,” as Bavinck observes… “From this it follows that in Reformation theology, grace cannot in any respect bear the character of a substance.
If grace is a spiritual substance infused into a person in order to perfect nature, rather than divine favor shown to those who are at fault, we have a perfect example of the contrast between ontological-metaphysical and ethical-covenanted construals of the problem. (pg. 194)
In contrast, Bavinck says, “the Reformation rejected this neoplatonic mysticism, returned to the simplicity of Holy Scripture, and consequently gained a very different concept of grace… Grace serves, not to take up humans into a supernatural order, but to free them from sin. Grace is opposed not to nature, only to sin. In its real sense, it was not necessary in the case of Adam before the fall but has only become necessary as a result of sin…The “physical” opposition between the natural and the supernatural yields to the ethical opposition between sin and grace.” In other words, grace does not elevate nature, but liberates it from its bondage to sin and death.
To be sure, Horton follows many other paths here, filling out this line of thinking. Several pages later, Horton continues:
Recognizing this danger (medieval ontology), Protestant orthodoxy walled off justification from any inward change, but then (with varying degrees of success) continued to appeal to traditional medieval categories for [to describe] regeneration and sanctification.
Although I affirm everything that they were after in the formulation, I am arguing that justification should be seen more clearly not merely as ontologically different from inner renewal, but also as the ontological source of that change (regeneration in both its narrower and broader senses). In that case, we need not reformulate a doctrine of regeneration as immediate and direct or even as subconscious and nontransformative, but treat justification as an illocutionary speech-act (verbum externum) that, when identified with the Spirit’s perlocutionary act of effectual calling, issues in repentance and faith. Developing this particular argument will be a central aim of the next chapter.
For Calvin, not only is justification entirely forensic; union with Christ is also regarded as first of all forensic and only consequently transformative.(198)
Unlike a human judge who can only call them as he sees them, God’s declaration [the double imputation of forgiveness for man’s sin and Christ’s righteousness back to man] … “creates the reality it declares.” “God’s declaration, in other words, is itself constitutive of that which is declared.
Once more we see the superiority of communicative and covenantal over purely causal and metaphysical grammars…. No less than God pronounced “Let there be…!” when there was nothing, Abram, the “father of many” while he was childless, Sarah fruitful while she was barren, a young woman pregnant while she was a virgin, God pronounces believers to be righteous while they are unrighteous. Thus, the entire reality of the new creation – not only justification but also renewal, and not only the renewal of the individual but also of the cosmos – is constituted by the covenantal speech of the Trinity. (201)
Of course, there is a lot more detail about this, but these are the basic outlines of Horton’s argument.