It is difficult to determine in what the second "Commonitorium" precisely differed from the first. The one preserved to us develops (chapters i-ii) a practical rule for distinguishing heresy from true doctrine, namely Holy Writ, and if this does not suffice, the tradition of the Catholic Church. Here is found the famous principle, the source of so much discussion particularly at the time of the Vatican Council, "Magnopere curandum est ut id teneatur quod ubique, quod semper, quod ab omnibus creditum est". Should some new doctrine arise in one part of the Church, Donatism for example, then firm adherence must be given to the belief of the Universal Church, and supposing the new doctrine to be of such nature as to contaminate almost the entirety of the latter, as did Arianism, then it is to antiquity one must cling; if even here some error is encountered, one must stand by the general councils and, in default of these, by the consent of those who at diverse times and in different places remained steadfast in the unanimity of the Catholic Faith (iii-iv). Applications of these principles have been made by St. Ambrose and the martyrs, in the struggle with the Donatists and the Arians; and by St. Stephen who fought against rebaptism; St. Paul also taught them (viii-ix). If God allows new doctrines, whether erroneous or heretical, to be taught by distinguished men, as for example Tertullian, Origen, Nestorius, Apollinaris, etc. (x-xix), it is but to test us. The Catholic admits none of these new-fangled doctrines, as we see from 1 Timothy 6:20-21 (20-22, 24). Not to remove all chance of progress in the faith, but that it may grow after the manner of the grain and the acorn, provided it be in the same sense, eodem sensu ac sententia; here comes the well known passage on dogmatic development. "crescat igitur. . ." (xxiii). The fact that heretics make use of the Bible in no way prevents them from being heretics, since they put it to a use that is bad, in a way worthy of the devil (xxv-xxvi). The Catholic interprets Scripture according to the rules given above (xxvii-xxviii). Then follows a recapitulation of the whole "Commonitorium" (xxix-xxx).
All this is written in a literary style, full of classical expressions, although the line of development is rather familiar and easy, multiplying digressions and always more and more communicative. The two chief ideas which have principally attracted attention in the whole book are those which concern faithfulness to Tradition (iii and xxix) and the progress of Catholic doctrine (xxiii). The first one, called very often the canon of Vincent of Lérins, which Newman considered as more fit to determine what is not than what is the Catholic doctrine, has been frequently involved in controversies. According to its author, this principle ought to decide the value of a new point of doctrine prior to the judgment of the Church. Vincent proposes it as a means of testing a novelty arising anywhere in a point of doctrine. This canon has been variously interpreted; some writers think that its true meaning is not that which answered Vincent's purpose, when making use of it against Augustine's ideas. It is hardly deniable that despite the lucidity of its formula, the explanation of the principle and its application to historical facts are not always easy; even theologians such as de San and Franzelin, who are generally in agreement in their views, are here at variance. Vincent clearly shows that his principle is to be understood is a relative and disjunctive sense, and not absolutely and by uniting the three criteria in one: ubique, semper, ab omnibus; antiquity is not to be understood in a relative meaning, but in the sense of a relative consensus of antiquity. When he speaks of the beliefs generally admitted, it is more difficult to settle whether he means beliefs explicitly or implicitly admitted; in the latter case the canon is true and applicable in both senses, affirmative (what is Catholic), and negative or exclusive (what is not Catholic); in the former, the canon is true and applicable in its affirmative bearing; but may it be said to be so in its negative or exclusive bearing, without placing Vincent completely at variance with all he says on the progress of revealed doctrine?
The "Commonitorium" has been frequently printed and translated. We may quote here the first edition of 1528 by Sichardus and that of Baluze (1663, 1669, 1684, Paris), the latter being the best of the three, accomplished with the help of the four known manuscripts; these have been used again in a new accurate collation by Rauschen, for his edition ("Florilegium patristicum", V, Bonn, 1906); a school-edition has been given by Julicher (Frieburg, 1895), and by Hurter (Innsbruck, 1880, "SS. Patrum opuscula selecta", IX) with useful notes.