Anselm of Canterbury
Throughout the "Summa," Aquinas cites Christian, Muslim, Hebrew, and Pagan sources, including, but not limited to: Christian Sacred Scripture, Aristotle, Augustine of Hippo, Avicenna, Averroes, Al-Ghazali, Boethius, John of Damascus, Paul the Apostle, Pseudo-Dionysius, Maimonides, Anselm of Canterbury, Plato, Cicero, and John Scotus Eriugena.