The feast of the Immaculate Conception, arriving as it does nine months before Our Lady's birthday (liturgical gestations are always perfect), is a favorite time for Franciscans to indulge our particular chauvinism by beating up on non-Franciscan theologians, St. Thomas in particular. This is because St. Thomas is said to have denied the Immaculate Conception, but only a generation or so later our own Bl. John Duns Scotus became a champion of the doctrine. Thus we Franciscans--sometimes infamous for our anti-intellectual tendencies--have a chance to be better than the great Angelic Doctor himself, and hence to all other smart people.
So this morning I decided I would actually read St. Thomas on the question, to see what he said. Truth be told, he does deny the Immaculate Conception in ST III. q. 27 II. (see the reply to 2nd objection in particular.) (1) He says that it would be derogatory to Christ as universal Savior to think that Mary never had any original sin. To be fair, we can hardly fault St. Thomas (whom I love, I'll admit it) for not holding a doctrine that would not be dogmatically defined for another 600 years. In order to answer the question of Mary's sanctification, of which he firmly believes, Thomas resorts (and as well he might) to Scriptural examples of holy individuals who were sanctified in utero, such as St. John the Baptist and the prophet Jeremiah, but not from the first moment of their conception. (See III. q.27 I, response.) (2)
In the end Thomas makes a good point. If Mary never knew original sin, what need did she have for Christ's salvation? Did Jesus Christ come to suffer for most? Didn't she need the salvation Christ won for us as well? The point provides an inadvertent reductio ad absurdum for the idea that salvation from sin was the principal purpose of the Incarnation. (It also shows us that the salvation Christ did indeed accomplish through his passover is not bound to time in a mechanical way.) It's not as if God made the world and when people sinned God then decided on the Incarnation as a means to our salvation. "The Incarnation was not plan B," as one of my favorite teachers liked to say. With God there is only plan A. Even if our first parents had not sinned, the Word would have come to us as the Incarnate Son of God, simply out of God's passionate desire to lift his creation to himself. That this required the salvation from sin we have through Christ's Passion is an extra, remedial gift. (Though it is a gift of inestimable value, which we celebrate each day in the sacrifice of the Mass.) After all, we could never say that sin controls how God behaves.
Hail Mary, full of grace, the Immaculate Conception and Queen of the Order of Friars Minor.
(1) "Ad secundum dicendum quod, si nunquam anima beatae virginis fuisset contagio originalis peccati inquinata, hoc derogaret dignitati Christi, secundum quam est universalis omnium salvator."
(2) "Respondeo dicendum quod de sanctificatione beatae Mariae, quod scilicet fuerit sanctificata in utero, nihil in Scriptura canonica traditur, quae etiam nec de eius nativitate mentionem facit. Sed sicut Augustinus, de assumptione ipsius virginis, rationabiliter argumentatur quod cum corpore sit assumpta in caelum, quod tamen Scriptura non tradit; ita etiam rationabiliter argumentari possumus quod fuerit sanctificata in utero. Rationabiliter enim creditur quod illa quae genuit unigenitum a patre, plenum gratiae et veritatis, prae omnibus aliis maiora gratiae privilegia accepit, unde legitur, Luc. I, quod Angelus ei dixit, ave, gratia plena. Invenimus autem quibusdam aliis hoc privilegialiter esse concessum ut in utero sanctificarentur, sicut Ieremias, cui dictum est, Ierem. I, antequam exires de vulva, sanctificavi te; et sicut Ioannes Baptista, de quo dictum est, Luc. I, spiritu sancto replebitur adhuc ex utero matris suae. Unde rationabiliter creditur quod beata virgo sanctificata fuerit antequam ex utero nasceretur."