Objection 1. It would seem that the articles of faith have not increased in course of time. Because, as the Apostle says (Hebrews 11:1), "faith is the substance of things to be hoped for." Now the same things are to be hoped for at all times. Therefore, at all times, the same things are to be believed.
Objection 2. Further, development has taken place, in sciences devised by man, on account of the lack of knowledge in those who discovered them, as the Philosopher observes (Metaph. ii). Now the doctrine of faith was not devised by man, but was delivered to us by God, as stated in Ephesians 2:8: "It is the gift of God." Since then there can be no lack of knowledge in God, it seems that knowledge of matters of faith was perfect from the beginning and did not increase as time went on.
Objection 3. Further, the operation of grace proceeds in orderly fashion no less than the operation of nature. Now nature always makes a beginning with perfect things, as Boethius states (De Consol. iii). Therefore it seems that the operation of grace also began with perfect things, so that those who were the first to deliver the faith, knew it most perfectly.
Objection 4. Further, just as the faith of Christ was delivered to us through the apostles, so too, in the Old Testament, the knowledge of faith was delivered by the early fathers to those who came later, according to Deuteronomy 32:7: "Ask thy father, and he will declare to thee." Now the apostles were most fully instructed about the mysteries, for "they received them more fully than others, even as they received them earlier," as a gloss says on Romans 8:23: "Ourselves also who have the first fruits of the Spirit." Therefore it seems that knowledge of matters of faith has not increased as time went on.
On the contrary, Gregory says (Hom. xvi in Ezech.) that "the knowledge of the holy fathers increased as time went on... and the nearer they were to Our Savior's coming, the more fully did they received the mysteries of salvation."
I answer that, The articles of faith stand in the same relation to the doctrine of faith, as self-evident principles to a teaching based on natural reason. Among these principles there is a certain order, so that some are contained implicitly in others; thus all principles are reduced, as to their first principle, to this one: "The same thing cannot be affirmed and denied at the same time," as the Philosopher states (Metaph. iv, text. 9). On like manner all the articles are contained implicitly in certain primary matters of faith, such as God's existence, and His providence over the salvation of man, according to Hebrews 11: "He that cometh to God, must believe that He is, and is a rewarder to them that seek Him." For the existence of God includes all that we believe to exist in God eternally, and in these our happiness consists; while belief in His providence includes all those things which God dispenses in time, for man's salvation, and which are the way to that happiness: and in this way, again, some of those articles which follow from these are contained in others: thus faith in the Redemption of mankind includes belief in the Incarnation of Christ, His Passion and so forth.
Accordingly we must conclude that, as regards the substance of the articles of faith, they have not received any increase as time went on: since whatever those who lived later have believed, was contained, albeit implicitly, in the faith of those Fathers who preceded them. But there was an increase in the number of articles believed explicitly, since to those who lived in later times some were known explicitly which were not known explicitly by those who lived before them. Hence the Lord said to Moses (Exodus 6:2-3): "I am the God of Abraham, the God of Isaac, the God of Jacob [Vulgate: 'I am the Lord that appeared to Abraham, to Isaac, and to Jacob']... and My name Adonai I did not show them": David also said (Psalm 118:100): "I have had understanding above ancients": and the Apostle says (Ephesians 3:5) that the mystery of Christ, "in other generations was not known, as it is now revealed to His holy apostles and prophets."
Reply to Objection 1. Among men the same things were always to be hoped for from Christ. But as they did not acquire this hope save through Christ, the further they were removed from Christ in point of time, the further they were from obtaining what they hoped for. Hence the Apostle says (Hebrews 11:13): "All these died according to faith, not having received the promises, but beholding them afar off." Now the further off a thing is the less distinctly is it seen; wherefore those who were nigh to Christ's advent had a more distinct knowledge of the good things to be hoped for.
Reply to Objection 2. Progress in knowledge occurs in two ways. First, on the part of the teacher, be he one or many, who makes progress in knowledge as time goes on: and this is the kind of progress that takes place in sciences devised by man. Secondly, on the part of the learner; thus the master, who has perfect knowledge of the art, does not deliver it all at once to his disciple from the very outset, for he would not be able to take it all in, but he condescends to the disciple's capacity and instructs him little by little. It is in this way that men made progress in the knowledge of faith as time went on. Hence the Apostle (Galatians 3:24) compares the state of the Old Testament to childhood.
Reply to Objection 3. Two causes are requisite before actual generation can take place, an agent, namely, and matter. On the order of the active cause, the more perfect is naturally first; and in this way nature makes a beginning with perfect things, since the imperfect is not brought to perfection, except by something perfect already in existence. On the other hand, in the order of the material cause, the imperfect comes first, and in this way nature proceeds from the imperfect to the perfect. Now in the manifestation of faith, God is the active cause, having perfect knowledge from all eternity; while man is likened to matter in receiving the influx of God's action. Hence, among men, the knowledge of faith had to proceed from imperfection to perfection; and, although some men have been after the manner of active causes, through being doctors of faith, nevertheless the manifestation of the Spirit is given to such men for the common good, according to 1 Corinthians 12:7; so that the knowledge of faith was imparted to the Fathers who were instructors in the faith, so far as was necessary at the time for the instruction of the people, either openly or in figures.
Reply to Objection 4. The ultimate consummation of grace was effected by Christ, wherefore the time of His coming is called the "time of fulness [Vulgate: 'fulness of time']" (Galatians 4:4). Hence those who were nearest to Christ, wherefore before, like John the Baptist, or after, like the apostles, had a fuller knowledge of the mysteries of faith; for even with regard to man's state we find that the perfection of manhood comes in youth, and that a man's state is all the more perfect, whether before or after, the nearer it is to the time of his youth.
Source: Summa Theologica (New Advent)