Objection 1. It would seem that those things that are of faith should not be divided into certain articles. For all things contained in Holy Writ are matters of faith. But these, by reason of their multitude, cannot be reduced to a certain number. Therefore it seems superfluous to distinguish certain articles of faith.
Objection 2. Further, material differences can be multiplied indefinitely, and therefore art should take no notice of them. Now the formal aspect of the object of faith is one and indivisible, as stated above (Article 1), viz. the First Truth, so that matters of faith cannot be distinguished in respect of their formal object. Therefore no notice should be taken of a material division of matters of faith into articles.
Objection 3. Further, it has been said by some [Cf. William of Auxerre, Summa Aurea] that "an article is an indivisible truth concerning God, exacting [arctans] our belief." Now belief is a voluntary act, since, as Augustine says (Tract. xxvi in Joan.), "no man believes against his will." Therefore it seems that matters of faith should not be divided into articles.
On the contrary, Isidore says: "An article is a glimpse of Divine truth, tending thereto." Now we can only get a glimpse of Divine truth by way of analysis, since things which in God are one, are manifold in our intellect. Therefore matters of faith should be divided into articles.
I answer that, the word "article" is apparently derived from the Greek; for the Greek arthron [Cf. William of Auxerre, Summa Aurea] which the Latin renders "articulus," signifies a fitting together of distinct parts: wherefore the small parts of the body which fit together are called the articulations of the limbs. Likewise, in the Greek grammar, articles are parts of speech which are affixed to words to show their gender, number or case. Again in rhetoric, articles are parts that fit together in a sentence, for Tully says (Rhet. iv) that an article is composed of words each pronounced singly and separately, thus: "Your passion, your voice, your look, have struck terror into your foes."
Hence matters of Christian faith are said to contain distinct articles, in so far as they are divided into parts, and fit together. Now the object of faith is something unseen in connection with God, as stated above (Article 4). Consequently any matter that, for a special reason, is unseen, is a special article; whereas when several matters are known or not known, under the same aspect, we are not to distinguish various articles. Thus one encounters one difficulty in seeing that God suffered, and another in seeing that He rose again from the dead, wherefore the article of the Resurrection is distinct from the article of the Passion. But that He suffered, died and was buried, present the same difficulty, so that if one be accepted, it is not difficult to accept the others; wherefore all these belong to one article.
Reply to Objection 1. Some things are proposed to our belief are in themselves of faith, while others are of faith, not in themselves but only in relation to others: even as in sciences certain propositions are put forward on their own account, while others are put forward in order to manifest others. Now, since the chief object of faith consists in those things which we hope to see, according to Hebrews 11:2: "Faith is the substance of things to be hoped for," it follows that those things are in themselves of faith, which order us directly to eternal life. Such are the Trinity of Persons in Almighty God [The Leonine Edition reads: The Three Persons, the omnipotence of God, etc.], the mystery of Christ's Incarnation, and the like: and these are distinct articles of faith. On the other hand certain things in Holy Writ are proposed to our belief, not chiefly on their own account, but for the manifestation of those mentioned above: for instance, that Abraham had two sons, that a dead man rose again at the touch of Eliseus' bones, and the like, which are related in Holy Writ for the purpose of manifesting the Divine mystery or the Incarnation of Christ: and such things should not form distinct articles.
Reply to Objection 2. The formal aspect of the object of faith can be taken in two ways: first, on the part of the thing believed, and thus there is one formal aspect of all matters of faith, viz. the First Truth: and from this point of view there is no distinction of articles. Secondly, the formal aspect of matters of faith, can be considered from our point of view; and thus the formal aspect of a matter of faith is that it is something unseen; and from this point of view there are various distinct articles of faith, as we saw above.
Reply to Objection 3. This definition of an article is taken from an etymology of the word as derived from the Latin, rather than in accordance with its real meaning, as derived from the Greek: hence it does not carry much weight. Yet even then it could be said that although faith is exacted of no man by a necessity of coercion, since belief is a voluntary act, yet it is exacted of him by a necessity of end, since "he that cometh to God must believe that He is," and "without faith it is impossible to please God," as the Apostle declares (Hebrews 11:6).
Source: Summa Theologica (New Advent)