I’m shocked to see people on this thread casting doubt on the claim made by HmanTheChicken that the early Christian Fathers and Doctors of the Church, including Origen (185-254), Augustine (354-430) and St. Thomas Aquinas (1225-1274), believed in a young Earth. I firmly believe in a 13.8-billion-year-old universe, but I have to say that HmanTheChicken is absolutely right on this one.
The arch-allegorist Origen vigorously defended “the Mosaic account of the creation, which teaches that the world is not yet ten thousand years old, but very much under that,” against the pagan skeptic, Celsus, in his book, Contra Celsus (Against Celsus) Book 1, chapter 19.
St. Augustine of Hippo criticized pagan myths of an old Earth, writing in his City of God, Book XII, Chapter 10: “They are deceived, too, by those highly mendacious documents which profess to give the history of many thousand years, though, reckoning by the sacred writings, we find that not 6000 years have yet passed.” Augustine, who favored the Septuagint over the Hebrew text of the Old Testament, held that the world was created around 5,600 B.C. What’s more, he held that the six days of Genesis 1 were in fact simultaneous and instantaneous, based on his reading of the deuterocanonical book of Sirach, chapter 18, verse 1: “He who lives eternally has made everything at the same time." (As it turned out, his Old Latin translation of the book was faulty: the original Greek text actually said that that God had made all things together.)
St. Thomas Aquinas approvingly quotes the testimony of St. Jerome in his Summa Theologica vol. I, q. 61, art. 3: “Six thousand years of our time have not yet elapsed; yet how shall we measure the time, how shall we count the ages, in which the Angels, Thrones, Dominations, and the other orders served God?” The question Aquinas is addressing is whether the angels were created before the world. Jerome thinks they were; Aquinas disagrees, and argues that they were created simultaneously with the world. What Aquinas does not question, however, is the young age of the world itself.
Look. We can argue ad nauseam about what the early Christian Fathers and Doctors of the Church would have thought, had they known what we know now. But no-one can reasonably doubt that in point of fact, they espoused young-Earth creationism. Cheers.