Réginald Marie Garrigou-Lagrange, O.P. is (or, I suppose I should say, "was") one of the leading Thomist theologians of the 20th century. In his Discourse II on Mary's Immaculate Conception, published in "The Mother of the Savior" (1948), Garrigou-Lagrange wrote:
In the final period of his career, when writing the Exposito super salutatione angelica----which is certainly authentic -----in 1272 or 1273, St. Thomas expressed himself thus: 'For she [the Blessed Virgin] was most pure in the matter of fault (quantum ad culpam) and incurred neither Original nor mental nor venial sin.'The problem is this:
The "neither original" in that quotation is an interpolation. Gibbings pointed that out long ago in his "Roman forgeries and falsifications" but you can see for yourself if you get a modern critical text of the work.
The Latin actually says "Ipsa enim purissima fuit et quantum ad culpam, quia ipsa virgo nec mortale nec veniale peccatum incurrit." ("For she [the Blessed Virgin] was most pure because the Virgin herself incurred neither mortal nor venial sin.")
What is especially shameful about this lie (perhaps I should be reluctant to call it a lie when Garrigou-Lagrange may simply have been working from a corrupted text, but it is hard to attribute ignorance of Thomas to a Thomist of his stature) is that the same work earlier explained:
"Sed Christus excellit beatam virginem in hoc quod sine originali conceptus et natus est. Beata autem virgo in originali est concepta, sed non nata." ("But Christ excels the Blessed Virgin in this, because he was conceived and born without original [sin]. Therefore, the Blessed Virgina was conceived in original [sin] but not born in it].")
No, Aquinas died believing that Mary was conceived in original sin. Garrigou-Lagrange is to be blamed for perpetuating a falsehood about Thomas and Taylor Marshall is to be blamed (much less, of course) for perpetuating Garrigou-Lagrange's error. Does that make Thomas a modern Protestant? Of course not. He disagreed with us on many matters, even about Mary.
How can you cash out this fact? Well, Rome insists today that you must believe in the immaculate conception of Mary. The immaculate conception of Mary is not taught in Scripture and it was not taught by any father prior to Augustine. It was denied by numerous men who were or became bishops of Rome. Even Garrigou-Lagrange states (a little above his attempt to resuscitate Thomas for the immaculatist position):
The Council of Trent (Denz., 792) declares, when speaking of Original Sin which infects all men, that it does not intend to include the Blessed and Immaculate Virgin Mary. In 1567 Baius is condemned for having taught the contrary (Denz., 1073). In 1661 Alexander VII affirmed the privilege, saying that almost all Catholics held it, though it had not yet been defined (Denz., 1100). Finally, on December 8th, 1854, we have the promulgation of the solemn definition (Denz., 1641).Thomas and the others help to show that Rome's demand that we believe in Mary's immaculate conception is really a demand for us to have implicit faith in the church of Rome. The dogma cannot be established from Scripture, it cannot be established from the fathers of the first three centuries, and it is opposed to the testimony of folks like Thomas Aquinas, who could hardly have been unaware of an apostolic tradition of an immaculate conception, if one existed.
It must be admitted that in the 12th and 13th centuries certain great doctors, as, for example, St. Bernard,  St. Anselm,  Peter Lombard,  Hugh of St. Victor,  St. Albert the Great,  St. Bonaventure,  and St. Thomas Aquinas appear to have been disinclined to admit the privilege.
29. Epist. ad canonicos Lugdunenses.
30. De conceptione virginali.
31. In III Sent., dist. 3.
32. Super Missus est.
33. Item Super Missus est.
34. In III Sent., dist. 3, q. 27.
Therefore, Rome is claiming the ability to simply define dogma that cannot be proven from Scripture or Tradition (History) and make that dogma so central to the faith that to deny is to - well - hear for yourself:
Hence, if anyone shall dare--which God forbid!--to think otherwise than as has been defined by us, let him know and understand that he is condemned by his own judgment; that he has suffered shipwreck in the faith; that he has separated from the unity of the Church; and that, furthermore, by his own action he incurs the penalties established by law if he should dare to express in words or writing or by any other outward means the errors he think in his heart.(Ineffabilis Deus - 1854)
That is sola ecclesia for you. If you implicitly trust Rome, the testimony of about 10 bishops of Rome and about half a dozen doctors of the church (Gregory the Great, Albert, Bernard, Aquinas, Anselm, and Bonaventure) will not matter. Yet, if you will critically consider Rome's claims, perhaps this issue of the Immaculate Conception can help you to see that Rome's claims about itself are false. She had no right to define the dogma of the Immaculate Conception, and no good reason to think it true. She cannot establish it from Scripture and it is not an apostolic tradition.