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The Church believed from the beginning but it was codified until your citation. Why do you think the Church rejoiced at the proclamation? If this had been some unknown doctrine, there would have been widespread scandal. As it was, what had already been known was now proclaimed inspired and eternal truth.

As to St Thomas Aquinas's position on the matter, I offer the following from the Summa Theologica:

“I answer that, God so prepares and endows those, whom He chooses for some particular office, that they are rendered capable of fulfilling it, according to 2 Cor. 3:6: ‘(Who) hath made us fit ministers of the New Testament.’ Now the Blessed 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 (Lk. 1:30,31): ‘Thou hast found grace with God: 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 Prov. 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 Cor. 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 Cor. 1:24) dwelt in her, not only in her soul but in her womb. And it is written (Wis. 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 (Cant 4:7) is fulfilled: ‘Thou art all fair, O my love, and there is not a spot in thee,’ etc. “

Corinthians is used by Calvinists in support of their doctrine of Predestination, and the word itself actually appears in 1 Corinthians 2:7. Being born sinless in order to give birth to Jesus Christ is what, if not the same? Yet your church rejects what was taught by the Apostle Paul on the one hand, yet aggrandizes it beyond recognition on the other. Internally inconsistent theology.