“…All that is true, by whomsoever it has been said, has its origin in the Spirit…”
***- Saint Thomas Aquinas (1225-1274), Catholic theologian, mystic and Doctor of the Church ***
In saying this Saint Thomas was merely attesting to this Sacred Tradition whic had been spoken of much earlier by the Fathers, for example Saint Justin Martyr and Augustine (as you quoted above):
“…All truth, wherever it is found, belongs to us as Christians and is Christian truth…”
***- Saint Justin Martyr (AD 100–165), Early Catholic Church Father ***
This is why Pope John Paul II once said, “You speak of many religions. Instead I will attempt to show the common fundamental element and the common root of these religions…From the beginning, Christian Revelation has viewed the spiritual history of man as including, in some way, all religions, thereby demonstrating the unity of humankind with regard to the eternal and ultimate destiny of man. The Church sees the promotion of this unity as one of its duties.”
And as Cardinal Nicholas of Cusa wrote in the 1400s, there is only one Wisdom and underlying religion in the diversity of faiths:
“…With many groanings I beseeched the Creator of all, because of His kindness, to restrain the persecution that was raging more fiercely than usual on account of the difference of faith between the religions…We praise our God, whose mercy rules over all His works and who alone has the power to bring it about, that such a great diversity of religions would be brought together in one harmonious peace. We, who are His work, cannot disobey His direction. Nevertheless we request instruction, as to how this unity of religion can be introduced by us…You will find that not another faith but the one and the same faith is presupposed everywhere…There can only be one wisdom. For if it were possible that there be several wisdoms, then these would have to be from one. Namely, unity is prior to all plurality…Even though you acknowledge diverse religions, you all presuppose in all of this diversity the one, which you call wisdom…Therefore, come to our aid you who alone are able. For this rivalry [among religions] exists for sake of you, whom alone they revere in everything that all seem to worship. For each one desires in all that he seems to desire only the good which you are; no one is seeking with all his intellectual searching for anything else than the truth which you are. For what does the living seek except to live? What does the existing seek except to exist? Therefore, it is you, O God, the giver of life and being, who is being sought in different religions in different ways, and who are named with different names because as you are you remain unknown and ineffable to all…Therefore, do not hide Yourself any longer, O Lord. Be propitious, and manifest Your face; and all peoples will be saved, who no longer will be able to desert the Source of life and its sweetness, once having foretasted even a little thereof. For no one departs from You except because He is ignorant of You. If You will deign to do the foregoing, the sword will cease, as will also the malice of hatred and all evils; and all will come to know that there is only one religion in the variety of faiths…”
***- Cardinal Nicholas of Cusa (1401 –1464), De Pace Fidei, Catholic mystic and highest cleric under the Pope in his day ***
I attended a Catholic High School and of course we had religion every day. The new priest that taught us told us to leave our religon book in our locker because we were really going to learn our faith.