First, Richard Sibbes, To the Christian Reader, (prefixed to John Smith’s An Exposition of the Creed: cited in A.B. Grosart, Memoir of Richard Sibbes, D.D., in v.1 of the BOT reprint, p.CIII)
Though we are to believe circumstance as well as the thing itself, yet not with the same necessity of faith, as it is more necessary to believe that Christ was crucified than that it was under Pontius Pilate; though when any circumstance is revealed we ought to believe it, and to have a preparation of mind to believe whatsoever shall be revealed. Yet in the main points this preparation of mind is not sufficient, but there must be a present and an expressed faith. We must know, as in the law, he that breaketh one commandment breaketh all, because all come from the same authority; so, in the grounds of faith, he that denies one in the true sense of it denies all, for both law and faith are copulatives. The singling out of anything is contrary to the obedience of faith. Fides non eligit objectum.
Second, Heinrich Heppe, Reformed Dogmatics
�Hence it follows (1), that the distinction between fundamental and derivative articles of doctrine is well worth observation. The doctrinal propositions in which the real foundation of doctrine is expressed and expounded have a higher and more essential meaning than those which do not impinge upon it directly.�All Reformed dogmaticians discuss this distinction with peculiar interest. E.g. Voetius II p. 513: “The first hypothesis is, that everything that occurs in Scripture is not equally necessary to saving faith or to Church union and communion, or needs to be taught the faithful and inculcated upon them with a like necessity. This we gather from 1 Cor. 3:10,12,15; Phil. 3:15-16; 2 Tim. 1:13; Tit.1; 1 Tim. 6:3. There is the additional reason that as in all disciplines so in the Scripture the essentials and oiketa of religion, or the axioms or precepts are to be distinguished from the commentaries upon them.�p. 531: These (fundamental) articles are the principal theses in the separate dogmatic heads of the Christian catechism; or they are the common ennoiai and aphorisms of Christian doctrine, necessary for promoting and preserving the practice and profession of faith and holiness in the unity and society of the Church”.�Similarly Franz Turretin I, xiv, 5: “Although all truths which are revealed in Scripture are necessary to be believed as divine and infallible, they are not all equally necessary. Here we must accurately distinguish between the scope (amplitudo) and extension of the faith and its necessity. Not everything within the scope of faith is at once of its necessity”.