With the least shade of thought to sin allied;
Woman! above all women glorified,
Our tainted nature’s solitary boast.
Every Catholic-in-formation should memorize that text. If the suggestion itself would seem catechetically unsound to some people in charge of such things, I don't care. Oakes explains why one shouldn't care.
For one thing, IC could and should be appreciated by Protestants. Explicating a passage from Pius IX's bull defining the doctrine, Oakes points out:
Mary is the perfect example of sola gratia at work. Everything she later did and was given came from this first grace of predestination, won for her purely and entirely by the merits of Christ, not her own; and even those “merits” she “earned” came from the graces given her aboriginally, in view of her predestined status as the chosen Mother of the Savior. As Pius IX clearly asserts, by a venerable exegetical tradition dating from patristic times, she was predestined to be sinless when God spoke thus to the Serpent in the Garden of Eden after our first parents’ first sin: “I will put enmity between you and the woman and between your seed and her seed. He shall bruise your dead, and you shall bruise his heel” (Genesis 3:15).
I find that persuasive partly because it is so beautiful an instance of the analogia fidei. But that could be set aside as mere opinion—which is why, as a Catholic, I find the authority of the Magisterium sufficient of itself to call for assent to the doctrine of the IC (henceforth 'DIC' for short'). But those for whom no such factors, either severally or collectively, suffice to guarantee truth have objected to DIC. Here I want to consider two which, albeit from different Christian traditions, ultimately converge.
One is usually heard from Protestants, though by no means only from Protestants: DIC is nowhere to be found in Scripture or the confessions of the early Church. From that point of view, DIC can only appear a theological speculation, fueled historically by an (actually or potentially) idolatrous veneration of a strictly human person whom we have no basis in divine revelation for believing to be special beyond her obvious, privileged status as the one who bore Our Savior into the world. That objection does not raise any special difficulty about predestination, a Pauline doctrine that many Protestants profess in some-or-other strong sense. But the other objection I want to consider centers on what is taken to be the concept of predestination that DIC entails.
I have heard it said, mostly by some Orthodox but also by a few non-Orthodox, that the kind of predestination which Mary is presented by Pius IX as enjoying is incompatible with human freedom, so that she cannot be said to have freely consented to anything of salvific significance. If sound, that objection would show Catholic doctrine to be internally inconsistent. For the Catholic Church professes that Mary freely consented to the Incarnation; indeed, the substance of the Church's confession on this point itself entails that Mary's free consent was, in Oakes' words, "the great fulcrum-shift in the drama of salvation." And yet, it is said, the sort of predestination entailed by DIC made that consent inevitable before the fact, and therefore unfree.
The first, mostly Protestant objection is just a special case of the more general divergence between Protestantism and the two older Christian traditions about the role of Tradition in conveying special divine revelation to us. If Scripture is not merely a normative rule of faith and materially sufficient to convey such revelation, but is rather the only such rule and thus formally sufficient—i.e., if sola scriptura is true in a sense strong enough to exclude both Catholic and Orthodox accounts of Tradition—then the objection in question is decisive. One cannot simply deduce DIC, as a matter of mere logic, from Scripture or the confessions of the early Church; indeed, not even the Church says otherwise. But the if-clause is the salient premise, not the consequent; and for both intellectual and experiential reasons, I have never found that premise remotely persuasive. As I've explained elsewhere, the arguments for it cannot help reducing Christianity to subjective opinion rather than the proximate object of faith as a divine gift. And since the objection does not concern DIC in particular more than a host of other doctrines, I shall leave it aside for the moment.
The other objection, from human freedom, is more potent. What actually lends force to it is the causal circle that Oakes presents as a key basis for meditation and development of doctrine. Thus:
...Mary could not be kept free from sin except by the merits of Christ won on the cross; but of course Christ could not have entered history to save us by dying on the cross except by the free consent of Mary, whose free assent to the angel was a truly graced assent vouchsafed by the future death of her Son.
Now I agree with Oakes about the "dizzying implications" of that circle. Taking his cue from Ratzinger and von Balthasar, he notes that the circle must be affirmed and can be usefully explained, as von Balthasar did in the third volume of his Theo-Drama. But acknowledging and meditating on it does not answer the objection; it remains merely assumed that Mary's consent was both predestined and free. How can the objection be answered?
I propose seeing Mary's freedom as a sharing in Christ's, and thus as a key aspect in which she instanced how all the baptized are called to become "partakers of the divine nature" (2 Peter 1:4). As St. Maximus the Confessor showed, the human will of Jesus Christ was free even though he could not have sinned; if so, then in general, the exercise of human freedom need not entail the possibility of choosing in a manner contrary to God's positive will. Christ's human will freely chose to suffer and die—even though that, along with the Incarnation itself, had been eternally, unalterably, and in that sense necessarily decreed by the Trinity, and therefore by the person of the Son, who is solely and identically the person who is and acts as Jesus Christ. The retroactive causality of the Passion on Mary, in the form of making her kecharitomene beginning with the IC, was simply the way she was similarly decreed to enjoy a pre-eminent capacity to have and exercise the kind of freedom her Son did in undergoing the Passion. The grace with which she was filled at the IC was not, then, an overriding of her human freedom, but was rather a necessary condition for exercising the kind of freedom her Son was to exercise as a man and which made her own possible. That's the kind of freedom she was to exercise by consenting to the Incarnation.
Oakes presents the causal circle assumed here as, historically, the basis for that authentic development of doctrine which we now call the IC. Many conservative Protestants object to such "developments" on the ground that they go beyond Scripture and, merely by that fact, add to the deposit of faith what is not there. Many Orthodox object that, even if such "developments" are acceptable as theological opinions, dogmatizing them imposes more of a confessional burden than the common deposit warrants. But such objections do not address the substance of the Catholic Church's ongoing meditation on the Virgin; they merely question her authority to draw forth from the deposit of faith the treasures she claims to find there. It seems to me that the beauty radiating through the DIC is itself a reason to question the questioners.