St. Thomas affirming the Immaculate Conception!

[Geremia]

St. Thomas's views on the Immaculate Conception went through three phases.
^{cf. also ch. 2, art. 2, § "St. Thomas and the Immaculate Conception" of Mother of the Saviour and Our Interior Life by Fr. Réginald Garrigou-Lagrange, O.P.}

Phase 1: clear support of it
St. Thomas's clearest support of the Immaculate Conception is in his commentary (1252-6) on Peter Lombard's Sentences (Super Sent., lib. 1 d. 44 q. 1 a. 3 ad 3):

puritas intenditur per recessum a contrario: et ideo potest aliquid creatum inveniri quo nihil purius esse potest in rebus creatis, si nulla contagione peccati inquinatum sit; et talis fuit puritas beatae virginis, quae a peccato originali et actuali immunis fuit.
Purity is increased by withdrawing from its opposite: hence there can be a creature than whom no more pure is possible in creation, if it be free from all contagion of sin: and such was the purity of the Blessed Virgin who was immune from original and actual sin.

Phase 2: grappling with it
Summa Theologica III, written in 1272-3, contains the famous question 27 on the Sanctification of the Blessed Virgin, in which he seems to deny the Immaculate Conception.

Phase 3: return to his original position
Explanation of the Lord's Prayer, petition 5 (Lent 1273):

...beatae virgini, quae fuit plena gratiae, in qua nullum peccatum fuit.
...the Blessed Virgin, who was full of grace, in whom there was no sin.

Commentary on Psalm 18 (1272-3):

...beata virgine, quae nullam habuit obscuritatem peccati.
...the Blessed Virgin, who had no darkness of sin.

He preached in his On the Angelic Salutation (Lent 1273):

Ipsa (Virgo) omne peccatum vitavit magis quam alius sanctus, praeter Christum. Peccatum enim aut est originale, et de isto fuit mundata in utero; aut mortale aut veniale, et de istis libera fuit. ... Sed Christus excellit beatam virginem in hoc quod sine originali conceptus et natus est. Beata autem virgo in originali est concepta, sed non nata.
For She Herself avoided every sin, more holy than anyone after Christ. For sin is either original, and from this she was cleansed in the womb; or mortal or venial, and from these She was free. ... But Christ excelled the Blessed Virgin in this, that He was conceived and born without original (sin). Moreover the the Blessed Virgin was conceived in original (sin), but not born (in it).

^{The sense of "the Blessed Virgin was conceived in/with original sin (in/cum peccato originali)" is explained by a similar passage of his Compendium Theologiæ cap. 224 ("Sanctification of Christ's Mother") (1272-3), in which he seems to be referring to Sts. Anne's and Joachim's sexual act,* not to the Blessed Virgin's participation in Adam's sin (or lack thereof):
*(cf. St. Augustine's theory that the propagation of original sin is due to the concupiscence inherent in the sexual act after the Fall, On Marriage & Concupiscence ch. 27, and this)}

^{Nec solum a peccato actuali immunis fuit, sed etiam ab originali, speciali privilegio mundata. Oportuit siquidem quod cum peccato originali conciperetur, utpote quae ex utriusque sexus commixtione concepta fuit. Hoc enim privilegium sibi soli servabatur ut virgo conciperet filium Dei. Commixtio autem sexus, quae sine libidine esse non potest post peccatum primi parentis, transmittit peccatum originale in prolem. Similiter etiam quia si cum peccato originali concepta non fuisset, non indigeret per Christum redimi, et sic non esset Christus universalis hominum redemptor, quod derogat dignitati Christi. Est ergo tenendum, quod cum peccato originali concepta fuit, sed ab eo quodam speciali modo purgata fuit.
Mary was not only free from actual sin, but she was also, by a special privilege, cleansed from original sin. She had, indeed, to be conceived with original sin, inasmuch as her conception resulted from the commingling of both sexes. For the privilege of conceiving without impairment of virginity was reserved exclusively to her who as a virgin conceived the Son of God. But the commingling of the sexes which, after the sin of our first parent, cannot take place without lust,* transmits original sin to the offspring. Likewise, if Mary had been conceived without original sin, she would not have had to be redeemed by Christ, and so Christ would not be the universal redeemer of men, which detracts from His dignity. Accordingly we must hold that she was conceived with original sin, but was cleansed from it in some special way.}

^{*(cf. Psalm 50:7: "For behold I was conceived in iniquities; and in sins did my mother conceive me.")}

See:

• Pedro Lumbreras O.P., "Saint Thomas and the Immaculate Conception," Homiletic and Pastoral Review 24, no. 3 (1923): 253–63. Response: Hugolinus Storff, O.F.M., The Immaculate Conception: The Teaching of St. Thomas, St. Bonaventure and Bl. J. Duns Scotus on the Immaculate Conception of the Blessed Virgin Mary (San Francisco, Calif.: St. Francis Press, 1925).
• Norbert del Prado O.P., Divus Thomas et bulla dogmatica "Ineffabilis Deus" (Friburgi Helvetiorum: Ex typis Consociationis sancti Pauli, 1919).

[Kephapaulos]

I had also myself understood that St. Thomas simply grappled with how to explain the Immaculate Conception, but I think the Lord did not allow him to figure it out in order to preserve his humility and let the Franciscans at least have one claim to fame in theology. Nevertheless, the theology of St. Thomas still led to the definition of the dogma.

Information in this Wikipedia article might shed some light on the subject (even though it's Wikipedia): https://en.wikipedia.org/wiki/Immaculate_Conception

Along the lines of what you were saying, Geremia, about what St. Thomas meant in his explanation in the Summa Theologiae, here is an interesting passage concerning St. Bernard of Clairvaux on the subject from the Wikipedia article relating to that:

It seems to have been St Bernard of Clairvaux who, in the 12th century, explicitly raised the question of the Immaculate Conception. A feast of the Conception of the Blessed Virgin had already begun to be celebrated in some churches of the West. St Bernard blames the canons of the metropolitan church of Lyon for instituting such a festival without the permission of the Holy See. In doing so, he takes occasion to repudiate altogether the view that the conception of Mary was sinless. It is doubtful, however, whether he was using the term "conception" in the same sense in which it is used in the definition of Pope Pius IX. Bernard would seem to have been speaking of conception in the active sense of the mother's cooperation, for in his argument he says: "How can there be absence of sin where there is concupiscence (libido)?" and stronger expressions follow, showing that he is speaking of the mother and not of the child.[5](from:http://encyclopedia.jrank.org/I27_INV/IMMACULATE_CONCEPTION_THE.html)

The Wikipedia article on Abortion and the Catholic Church also has some information talking about the Fathers and the time of ensoulment: https://en.wikipedia.org/wiki/Abortion_and_the_Catholic_Church

I remember also seeing a video of Dr. Taylor Marshall on the subject, but he expressed that there was no 100% certainty that St. Thomas is the author of the work on the Angelic Salutation thought to be composed by him toward the end of his life on earth.  Here is the video: https://www.youtube.com/watch?v=FVe8TFpfttY (Especially go to 8:45 and take note at 10:30 and 11:15 in the video).

Beginning at about 10:51 or 10:52, reference was made to this quote:

...the Blessed Virgin did indeed contract original sin...
(Summa theologiae IIIa, q. 27, a. 2, ad 2)(http://dhspriory.org/thomas/summa/TP/TP027.html#TPQ27A2THEP1)

Was the quote taken of out context though?

[Geremia]

Along the lines of what you were saying, Geremia, about what St. Thomas meant in his explanation in the Summa Theologiae, here is an interesting passage concerning St. Bernard of Clairvaux on the subject

Yes, Fr. Storff, O.F.M., argues that St. Bernard, St. Bonaventure, and the majority of theologians at the time "denied" the Immaculate Conception. I wouldn't go so far as to say they denied it, but would argue more like Fr. Lumbreras, O.P., that there are several ways in which there can be an "immaculate conception" and that St. Thomas did not deny the sense of "immaculate conception" of Ineffabilis Deus.

[Geremia]

Not to split hairs, but St. Thomas is in agreement with the Church if we consider conception to be the two-fold act that it is. On the one hand, there is the physical conception of the body; on the other, there is the spiritual conception of the soul animating that body. Both of these conceptions (the conception of the body and soul) happen in exactly the same instant in time, but the conception of the body necessarily precedes the conception of the soul in the ontological order. God doesn't create the soul until the body is conceived, so in the order of creation the existence of the body is logically and ontologically prior to the existence of the soul, though they are both created in the same temporal instant. Therefore, it is entirely possible that our Lady was physically conceived in Original Sin but spiritually conceived exempt from it, such that, at the very instant the soul was infused into the body (the moment of conception), the body no longer manifested the effects of original sin, though it retained all of its physical characteristics.By the action of man, she is conceived in Original Sin and, hence, from this perspective, St. Thomas is correct is pointing out she needs to be redeemed as Scripture says all men do.  But by the action of God, her soul is conceived exempt from it and so she is properly conceived without sin as the Immaculate Conception. Thus, Mary could in a sense physically appear to be like everyone else (and thus not draw undue attention to herself) while being free from the debilitating effects of Original Sin, both spiritual and physical.

EDIT:

The Church speaks of "conception" as a temporal instant, in which case it truly was the Immaculate Conception. However, the philosophers (like St. Thomas) distinguished the temporal order from the ontological order. It is this order that a cause may precede an effect, which both happen at the same temporal instant. Perhaps St. Thomas' dilemma concerned the prevalent opinion of his generation concerning the time between conception and ensoulment, known as "quickening." It was commonly believed that the body was conceived for some time in the womb before the soul was created to animate that body.  Progress in biological research has since shown us that there is every reason to believe that ensoulment takes place at the very instant of conception, though logically posterior to it (since the infusion of the soul logically necessitates the prior existence of the body). The language "conception" in St. Thomas' era would have referred to simple biological conception, anticipating the infusion of the soul. But if they happen simultaneously in time then the conception is truly immaculate, though in the ontological order the body is conceived in sin but sanctified in the same instant by the immaculate soul, free from Original Sin.

(source)