Why didn't you accept the immaculate conception, St. Thomas?

Objection 3. Further, as it has been stated above, no feast is celebrated except of some saint. But some keep the feast of the Conception of the Blessed Virgin. Therefore it seems that in her very Conception she was holy; and hence that she was sanctified before animation. Reply to Objection 3. Although the Church of Rome does not celebrate the Conception of the Blessed Virgin, yet it tolerates the custom of certain churches that do keep that feast, wherefore this is not to be entirely reprobated. Nevertheless the celebration of this feast does not give us to understand that she was holy in her conception. But since it is not known when she was sanctified, the feast of her Sanctification, rather than the feast of her Conception, is kept on the day of her conception.   ST. 3. Q 27. A 2. ---------- God so prepares and endows those, whom He chooses for some particular office, that they are rendered capable of fulfilling it, according to 2 Corinthians 3:6: "(Who) hath made us fit ministers of the New Testament." Now theBlessed Virgin was chosen by God to be His Mother. Therefore there can be no doubt that God, by His grace, made her worthy of that office, according to the words spoken to her by the angel (Luke 1:30-31): "Thou hast found grace withGod: behold thou shalt conceive," etc. But she would not have been worthy to be the Mother of God, if she had ever sinned. First, because the honor of the parents reflects on the child, according to Proverbs 17:6: "The glory of children are their fathers": and consequently, on the other hand, the Mother's shame would have reflected on her Son. Secondly, because of the singular affinity between her and Christ, who took flesh from her: and it is written (2 Corinthians 6:15): "What concord hath Christ with Belial?" Thirdly, because of the singular manner in which the Son of God, who is the "Divine Wisdom" (1 Corinthians 1:24) dwelt in her, not only in her soul but in her womb. And it is written (Wisdom 1:4): "Wisdom will not enter into a malicious soul, nor dwell in a body subject to sins." We must therefore confess simply that the Blessed Virgin committed no actual sin, neither mortal nor venial; so that what is written (Canticles 4:7) is fulfilled: "Thou art all fair, O my love, and there is not a spot in thee," etc.   ST. 3. Q 27. A 4.