Formal reason of faith versus the Church's public external rule
Speak carefully and you may also distinguish faith's proper formal reason — taken simply and absolutely, as the Schools put it — from an external principle of faith, an outward rule for what is to be believed.
Faith's proper formal reason was, is, and always will be the same for every believer: God's revelation, however it comes — by God's immediate breath, through men, through angels, through Scripture, or outside Scripture. For every believer the standing formal reason of religious belief is this: because God revealed such things.
Yet God did not judge it enough to reveal privately to each believer whatever that person needed for salvation. To keep unity and communion in the Church, and to meet Satan's tricks, he always willed a common, external, public revelation to stand in the Church as the rule of faith — the measure of every dogma, and the court that ends every dispute about faith.
Before the law was given, that rule was the Patriarchs' voices and God's living oracles. After Moses' law was given and written, the Church's rule was that written law, with the revelations later made to the Prophets and likewise written down. After Christ's coming the New Testament enlarged the rule. So now, in the Christian Church, the rule of our faith is the Scripture of both Testaments.
Latin
XX. Praeterea sciendum est, accurate loquendo, distinctionem adhiberi posse inter fidei propriam, et, ut loquuntur Scholae, formalem rationem, si consideretur simpliciter et absolute; et inter externum quoddam fidei principium, sive regulam externam credendorum. Fidei propria et formalis ratio eadem fuit, est, et erit semper apud omnes fideles, nempe Dei revelatio, quocunque tandem modo illa revelatio fiat, sive immediato Dei afflatu, sive per homines, sive per Angelos, sive per Scripturam, sive extra Scripturam. Etenim fidelibus cunctis haec perpetua credendi res fidei et religionis ratio formalis est, nempe, quia Deus talia revelavit. At Deo visum non est satis fidelibus singulis privatim revelare quae ad salutem cujusque necessaria essent, sed ad unitatem et communionem in Ecclesia servandam, et ut obviam iretur Satanae praestigiis, Deus semper voluit in Ecclesia extare communem quandam, externam et publicam revelationem, quae esset fidei regula, ad quam exigenda essent omnia fidei dogmata, et ex qua omnis de rebus fidei contentio decidenda et terminanda esset. Talis fidei regula ante legem datam erant Patriarcharum voces, et viva Dei oracula. Post legem vero a Mose datam et scriptis mandatam, regula fidei in Ecclesia fuit ipsa lex Mosis scripta, et revelationes Prophetis postea factae, et scriptis quoque consignatae. Post Christi vero adventum, haec regula Novi Testamenti accessione auctior et amplior facta est. Itaque jam fidei nostrae in Ecclesia Christiana regula est Scriptura Veteris et Novi Testamenti.