Early Thomas Aquinas contra the Immaculate Conception

Thomas Aquinas is one of the most notable theologians of the medieval period.  One of Thomas' earlier works was his commentary on Peter Lombard's Sentences.  At Sentences, III, D3, Q1, Thomas considers "the Sanctification of the Blessed Virgin," and specifically, "Concerning the first question, two things are to be looked into: first, the time of her sanctification; [and] second, its effects." 

Article 1 asks whether Mary was sanctified "before her conception was completed."  Thomas then considers multiple question-ettes (questinculae).  

In his "Response to Quaestinucula 1," he concludes that "it should be said that there is no way she could have been sanctified in her parents, nor even in the very act of her conception."

In his "Response to Quaestincula 2," he concludes that "it should be said that the Blessed Virgin’s sanctification could not have occurred in a becoming way before the infusion of her soul, because she was not yet capable of grace, but also not in the very instant of its infusion, such that by grace infused into her at that time she would be preserved from incurring original sin. For Christ was unique in the human race in not needing redemption, for he is our head, but all share in needing to be redeemed through him. Now, this could not be if any other soul were found that had never been infected by the original stain. And therefore this was not granted to the Blessed Virgin or anyone other than Christ."

In his "Response to Quaestincula 3," he concludes that "it should be said that the Blessed Virgin was sanctified before her birth from the womb, which can be gathered from the fact that she above all the other saints was most pure from sin, as the text states, as the chosen mother of divine Wisdom, in which no defilement occurs, as it says in Wisdom 7:22. Whence, since this purity may be found to have existed in some people such that they were cleansed before their birth from the womb—such as John the Baptist, of whom we read in Luke 1:15, he will be filled with the Holy Spirit, even from his mother’s womb; and Jeremiah, of whom it says, before I formed you in the womb I knew you, and before you were born I consecrated you (Jer 1:5)—one should not doubt that this was bestowed on the Mother of God in a much more excellent way."

So, Thomas' position is fairly straightforward in that he he says she was sanctified in the womb before birth, but after the infusion of her soul into her formed body.  This contrasts with what was later defined to be dogma of the immaculate conception.