I answer that, As stated above (Articles [4],6), to faith those things in themselves belong, the sight of which we shall enjoy in eternal life, and by which we are brought to eternal life. Now two things are proposed to us to be seen in eternal life: viz. the secret of the Godhead, to see which is to possess happiness; and the mystery of Christ's Incarnation, "by Whom we have access" to the glory of the sons of God, according to Rm. 5:2. Hence it is written (Jn. 17:3): "This is eternal life: that they may know Thee, the . . . true God, and Jesus Christ Whom Thou hast sent." Wherefore the first distinction in matters of faith is that some concern the majesty of the Godhead, while others pertain to the mystery of Christ's human nature, which is the "mystery of godliness" (1 Tim. 3:16).
Now with regard to the majesty of the Godhead, three things are proposed to our belief: first, the unity of the Godhead, to which the first article refers; secondly, the trinity of the Persons, to which three articles refer, corresponding to the three Persons; and thirdly, the works proper to the Godhead, the first of which refers to the order of nature, in relation to which the article about the creation is proposed to us; the second refers to the order of grace, in relation to which all matters concerning the sanctification of man are included in one article; while the third refers to the order of glory, and in relation to this another article is proposed to us concerning the resurrection of the dead and life everlasting. Thus there are seven articles referring to the Godhead.
In like manner, with regard to Christ's human nature, there are seven articles, the first of which refers to Christ's incarnation or conception; the second, to His virginal birth; the third, to His Passion, death and burial; the fourth, to His descent into hell; the fifth, to His resurrection; the sixth, to His ascension; the seventh, to His coming for the judgment, so that in all there are fourteen articles.
Some, however, distinguish twelve articles, six pertaining to the Godhead, and six to the humanity. For they include in one article the three about the three Persons; because we have one knowledge of the three Persons: while they divide the article referring to the work of glorification into two, viz. the resurrection of the body, and the glory of the soul. Likewise they unite the conception and nativity into one article.