Thomas Aquinas, Compendium of Theology

Having reviewed the doctrines pertaining to the truth of the Christian faith, we should advert to the fact that all the teachings thus set forth are reduced to certain articles: twelve in number, as some think, or fourteen, according to others.

Faith has to do with truths that surpass the comprehension of reason. Hence, whenever a new truth incomprehensible to reason is proposed, a new article is required. One article pertains to the divine unity. For, even though we prove by reason that God is one, the fact that He governs all things directly or that He wishes to be worshiped in some particular way, is a matter relating to faith. Three articles are reserved for the three divine persons. Three other articles are formulated about the effects produced by God: creation, which pertains to nature; justification, which pertains to grace; and reward, which pertains to glory. Thus seven articles altogether are devoted to the divinity.

Concerning the humanity of Christ, seven more are proposed. The first is on the incarnation and conception of Christ. The second deals with the nativity, which involves a special difficulty because of our Lord's coming forth from the closed womb of the Virgin. The third article is on the death, passion, and burial; the fourth on the descent into hell; the fifth on the resurrection; the sixth on the ascension; and the seventh treats of Christ's coming for the judgment. And so there are fourteen articles in all.

Other authorities, reasonably enough, include faith in the three persons under one article, on the ground that we cannot believe in the Father without believing in the Son and also in the Holy Spirit, the bond of love uniting the first two persons. However, they distinguish the article on the resurrection from the article on eternal reward. Accordingly, there are two articles about God, one on the unity, the other on the Trinity. Four articles deal with God's effects: one with creation, the second with justification, the third with the general resurrection, and the fourth with reward. Similarly, as regards belief in the humanity of Christ, these authors comprise the conception and the nativity under one article, and they also include the passion and death under one article. According to this way of reckoning, therefore, we have twelve articles in all.

And this should be enough on faith.