St. Thomas Aquinas, although recognized as one of the greatest theologians of all time, did manage, however, to fall into error. Please understand that the purpose of showing this is not to demonize St. Thomas, nor is it to suggest he was a heretic, but simply to point out the importance, as Catholics, to measure all things against the infallible judgment of the Church, even though they may come from holy Doctors such as St. Thomas, who although a Catholic genius, was not infallible.
Here are a few errors contained in his Summa Theologica:
Summa, Tertia Pars, Q. 27, Art. 2, Reply to Obj. 2: “For Christ did not contract original sin in any way whatever, but was holy in His very Conception, according to Luke 1:35: "The Holy which shall be born of thee, shall be called the Son of God." But the Blessed Virgin did indeed contract original sin, but was cleansed therefrom before her birth from the womb.”
This is in direct opposition (and is therefore overruled by) to the later definition of the Immaculate Conception, by Pope Pius IX.
Pope Pius IX, Ineffabilis Deus (The Immaculate Conception), 1854 , ex cathedra: “We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.”
Summa, Supplement (Appendix 1), Q. 1, Art. 2, Reply to Obj 5: "Although unbaptized children are separated from God as regards the union of glory, they are not utterly separated from Him: in fact they are united to Him by their share of natural goods, and so will also be able to rejoice in Him by their natural knowledge and love."
St. Thomas teaches above that infants who have died without baptism will be united to God and be able to rejoice in Him, despite the dogmatic truth that such children (being in original sin alone) are in fact condemned to the hell of the damned, which means they are separated from God.
Pope Eugene IV, Council of Florence, 1439, ex cathedra: "But the souls of those who depart this life in actual mortal sin, or in original sin alone, go down straightaway to hell to be punished, but with unequal pains."
The Summa Theologica, though mostly reliable, is an excellent example of why correct theology regards the solemn definitions of the Extraordinary Magisterium as the standard against which all doctrine is to be measured, which is also attested to by Pope Benedict XIV:
Pope Benedict XIV, Apostolica Constitutio, #6, June 26, 1749: "The Church’s judgment is preferable to that of a Doctor renowned for his holiness and teaching."
Thankfully, St. Thomas himself also recognized this important truth, and would therefore have humbly submitted to the definitions of Pius IX and Eugene IV:
St. Thomas Aquinas, Summa Theologica, Secunda Secundae Partis, Q. 10, A. 12: “The custom of the Church has very great authority and ought to be jealously observed in all things, since the very doctrine of Catholic doctors derives its authority from the Church. Hence we ought to abide by the authority of the Church rather than by that of an Augustine or a Jerome or of any doctor whatever.”
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