Sometimes, and for reasons that honestly escape me, certain Protestants will attempt to say that St. Thomas was “one of them:” Not that he was literally Protestant of course, for that would be anachronistic, but rather some folks claim that Aquinas’ theology is rather more of the Protestant kind than of the Catholic kind. The apparent (?) intent of this attempt to drive a wedge between the Church and her greatest theologian is to bolster the Protestant claim that the Church abandoned orthodoxy at some point in the late Middle Ages, and her alleged discordance with Aquinas proves this.
Don’t shoot the messenger; I merely report what I have seen on more than one occasion on the interwebs.
Speaking purely anecdotally, I can say with complete confidence that the claim is erroneous. I have read enough of Aquinas and enough Protestant theology to be entirely sure that St. Thomas is about as Protestant as corn flakes. Here is an example of the sort of thing that I mean. In ST I-II q. 81 a.5, Aquinas addresses the question of whether Eve’s sin alone (without Adam’s) would have been a sufficient condition for the Fall (and particularly for original sin to be passed along to the entire human race). One of the objections proffered (the third) claims that St. John of Damascus asserted Mary needed to be purified (and therefore, the objection continues, Eve’s sin would have been sufficient to cause the Fall and transmit original sin because Eve would not have needed to be purified if she could not transmit original sin herself: the difficulty here being twofold, in that she would have been tainted with sin herself while bearing the Lord in her womb and would have transmitted original sin to Him). Aquinas responds to this objection thusly (ad 3):
This prevenient purification in the Blessed Virgin was not needed to hinder the transmission of original sin, but because it behoved the Mother of God to shine with the greatest purity. For nothing is worthy to receive God unless it be pure, according to Ps. 92:5: Holiness becometh Thy House, O Lord. (emphasis in original)
I think it is fairly obvious that there is very little if anything here that is Protestant rather than Catholic. I have never met a Protestant who held to anything remotely like the sinlessness of the Blessed Virgin as it is taught by the Catholic Church (and by Aquinas in this passage). Furthermore, St. Thomas wields Scripture here in ways that are utterly foreign to the Protestant’s grammatical-historical approach to hermeneutics.
Examples could be multiplied, but I recently came across this in the Summa Theologiae again and realized how very un-Protestant it is. But this should not be surprising, because Aquinas was no proto-incipient-Protestant. He was Catholic.