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C.S. Lewis once said, “It’s a good rule after reading a new book never to allow yourself another new one till you have read an old one in between. If that is too much for you, you should at least read one old one to three new ones.... Every age has its own outlook. It is especially good at seeing certain truths and especially liable to make certain mistakes. We all therefore need the books that will correct the characteristic mistakes of our own period.... None of us can fully escape this blindness, but we shall certainly increase it, and weaken our guard against it, if we read only modern books....The only palliative is to keep the clean sea breeze of the centuries blowing through our minds and this can only be done by reading old books.” 
This principle becomes more evident in our own time. When we peer into those old books of each of our respective traditions, we see something fuller, deeper, more precise than those books of our day. Books today are beggarly in comparison to those of old. They are treasures that have been uncovered. They provide paradigm-shifting perspectives never thought of. The work of our Theological age is not providing “new,” “original,” or “fresh” perspectives as many have taught. Instead, the labor of our age is to labor to uncover those works that have been lost before we even think of building something new.
It is in this spirit that I write this series. Justification has taken a central role in polemical theology over the last five centuries due to the Reformation. Each has taken its narrowly defined side. Before then, Justification took a relatively minor role in the system of one’s theology. This series seeks to show the thought of one of Theology’s brightest lights before the polemics that arose around the Reformation. What can we learn from him? Did he take a side in our modern debate? What did he emphasize that we have missed?
What is Grace?
The first question one must ask when covering the topic of “the necessity of grace” is the question of “What is grace?” This question takes the reader back to bitter debates between Romanists and Protestants, specifically, the question of imputation vs. infusion in justification.  St. Thomas speaks clearly to this debate.
Foundational to his argument for imputation is Divine love. God’s love is an eternal act whereby He loves us. The effect of this eternal act is temporal in that He affects peace and union with us. The bringing us up into union with Him and the effecting of peace requires that there is an infusion of faith and charity into us. If not, we would not be capable of this union, and we would not be active towards this union (the action of which on both sides is necessary for union)
There is the extra nos and eternal reality of the Divine love and then its’ intra nos effect, which is the infusion of grace as an overflowing of this love. God’s eternal disposition of love is not impotent; rather, it is powerful and glorious, bringing about the effects of his will. The painful action is not only stopped but the wound is healed by Divine love that we may have true union with him, in peace.
St. Thomas writes, “by sinning a man offends God...an offense is remitted to anyone, only when the soul of the offender is at peace with the offended. Hence sin is remitted to us, when God is at peace with us, and this peace consists in the love whereby God loves us. Now God’s love, considered on the part of the Divine act, is eternal and unchangeable; whereas, as regards the effect it imprints on us, it is sometimes interrupted, inasmuch as we sometimes fall short of it and once more require it. Now the effect of the Divine love in us, which is taken away by sin, is grace, whereby a man is made worthy of eternal life, from which sin shuts him out. Hence we could not conceive the remission of guilt, without the infusion of grace.” 
Further, he writes, “As God’s love consists not merely in the act of the Divine will but also implies a certain effect of grace...so likewise, when God does not impute sin to a man, there is implied a certain effect in him to whom the sin is not imputed; for it proceeds from the Divine love, that sin is not imputed to a man by God.” 
This is also where merit comes into the discussion, another touchy topic between the Reformation and counter-Reformation. St. Thomas teaches that there are specific meritorious actions that result from grace. How can there be merit that results from grace? One may ask. The answer comes back to the definition of grace that St. Thomas gives. Notice earlier when he wrote, “Now the effect of the Divine love in us, which is taken away by sin, is grace, whereby a man is made worthy of eternal life, from which sin shuts him out.”  This is all that merit is for St. Thomas. We “merit” or “are made worthy” for eternal life in that we have been infused with faith, hope, and charity. In St. Thomas’ scheme, it is natural to speak of “merit” in this way, that is, as an effect of God’s love, where sin is taken away, and there is a motion from sin to justice. Of course, in this scheme, we “merit” eternal life in the sense that God has fulfilled in us the conditions of eternal life and taken away “the sin which shuts him out.” 
The Necessity of Infused Grace
St. Thomas goes on, in De Veritate, to prove the necessity of this infused grace. He begins by defining the nature of sin. There are two elements to sin, “the turning away from something and the turning towards something.”  We turn away from God and towards lesser loves. As St. Augustine taught in De Doctrina Christiana, we love things for their sake and not in reference to God.
St. Thomas teaches that the forgiveness of sin does not have reference to the second, turning towards something, but the guilt is found in turning away from God. It is not enough to only turn back to God; the guilt and damage incurred from turning away are still present. For this turning away to be forgiven, there is both a negative and a positive aspect. The will must be turned to God, and the damage incurred from the former turning away must be healed by grace.
To illustrate what St. Thomas is getting at here, it is as if we were supposed to lift something with our hands. We take our hands from the proper act of lifting and put them into a fire. It is not enough to take our hands out of the fire and begin lifting again. Our hands will be damaged beyond repair from being in a fire. The turning from and the effects of this act need to be healed for the lift to occur as initially intended. So also, our sin and the effect of our sin need to be healed for true union with God to take place. It is not enough for us to turn in faith to God if we are unfit instruments to have union with him. It is as if we were trying to fill a cup with holes in it. The water can flow in super-abundance (union and peace with God) towards the cup (us), but the holes must be patched (infused grace) before the water (union and peace with God) can flow into the cup (us).
Turning Away, Offence, and Effects
St. Thomas closes in his exposition by giving three more arguments from the nature of sin to argue for the necessity of infusion. First, from the nature of turning away. St. Thomas writes, “the turning away is from the unchangeable good, which the person could have possessed but in regard to which he has made himself impotent; otherwise the turning away would not be culpable. The turning away in question cannot, then, be removed unless there is brought about a union with the unchangeable good from which the man withdrew by his sin. But this union is effected only by means of grace, by which God dwells in souls and the soul cleaves to God by the love of charity. The healing of this turning away, accordingly, requires the infusion of grace and charity, just as the healing of blindness requires the restoration of the power of sight.” 
Second, from the nature of the offense. The offense is not turning away from a mere creature, as if we went from loving calzones more than pizza; rather, it is the supreme perversion of loving a creature more than the creator. St. Thomas writes, “Since God infinitely surpasses a creature, one who sins mortally will have offered to God an infinite offense from the point of view of the dignity of Him who is insulted...Human strength is accordingly incapable of blotting out this offense; the good offices of divine grace are required.” 
Third, from the effects of sin. Sin takes away the hope of glory. The means of arriving at glory is by grace; therefore, we need to be given grace for sin to be remitted.
In conclusion. St. Thomas here argues for the necessity of infused grace in the act of justification. While this is not certainly going to convince a Protestant of the Roman doctrine (this was not even St. Thomas’s aim), it gives us brilliant insight into an exposition of the doctrine presented. While we may criticize the lack of biblical exposition on the part of St. Thomas and the lack of interaction with the modern protestant position (which was unknown to St. Thomas), this exploration is still of great value to any reader.
 C.S. Lewis, Introduction in St. Athanatius, On the Incarnation
 I am aware that Protestants also believe in infused grace as is clear from Westminster Larger Catechism Q. 77, “Although sanctification be inseparably joined with justification, yet they differ, in that God in justification imputeth the righteousness of Christ; in sanctification of his Spirit infuseth grace, and enableth to the exercise thereof; in the former, sin is pardoned; in the other, it is subdued: the one doth equally free all believers from the revenging wrath of God, and that perfectly in this life, that they never fall into condemnation the other is neither equal in all, nor in this life perfect in any, but growing up to perfection.” The chief difference is that the infusion of grace is not typically included under the act of justification.