I. The question concerning fundamental articles is difficult and important. It has been discussed by many who have erred both in defect and in excess. The Socinians err in defect who admit very few fundamentals (and those only practical, the theoretical being almost entirely set aside) so that they may teach that the settling of the differences in religion is easy since they relate more to theological conclusions and to the dogmas of the schools than to fundamental articles of faith (which are both few in number and held substantially by both sides). Under this pretext they take away from fundamentals the principal doctrines of faith: as the doctrine concerning the Holy Spirit, the Trinity, the person of Christ, the satisfaction, etc. To this class the Arminians evidently belong who reduce fundamentals to those heads which are placed beyond dispute among almost all Christians and are contained in these three: faith in the divine promises, obedience to the divine precepts and a due reverence for the Scriptures.
II. Those who err in excess are both the papists who are impudent enough often to declare as fundamental their own hay and stubble and whatever the Romish church teaches; and the more strict Lutherans who (to render a union with us more difficult) extend fundamentals more widely than is just, turn almost every error into a heresy and make necessary those things which are indifferent so as more easily to prove that we differ on fundamentals.
III. The orthodox hold the mean between both. As they necessarily build upon some fundamentals, so they neither restrict them too closely, nor extend them too far.
IV. As in a house that is the foundation which has such a position that the house can neither be erected nor stand without it, so in religion that is the foundation upon which the whole of religion depends and it standing, religion stands; removed, religion falls. It is used in two senses: either simply and personally (applied to Christ the foundation of all salvation upon which the church and religion are built as upon a firm and immovable rock, Mt. 16:18; 1 Pet. 2:6, 7; 1 Cor. 3:11—“for other foundation can no man lay than that is laid, which is Jesus Christ”); or comprehensively and naturally for the fundamental truth which all are required to believe and which is therefore called the foundation of faith. But this may be either widely extended to the first rudiments of the Christian religion (which were taught the catechumens for initiation and are called by the apostle the foundation [themeliou], or the principles [archēs]): such as repentance from dead works, faith toward God, the doctrine of baptisms, the laying on of hands, the resurrection of the dead and eternal judgment (Heb. 6:1, 2). These principles however have not an equal degree of necessity, some being necessary primarily and by themselves and others secondarily only and by reason of some other thing. In this sense, fundamental articles of religion belong to the decalogue, the Apostles’ Creed, the Lord’s Prayer, the sacraments and the power of the keys because they contain the doctrine of salvation as necessary and fundamental without which we cannot receive the rest. Or strictly, it denotes the essential doctrines of Christianity of which the theory (theōria) and practice is necessary simply as to the thing itself; or which are simply and absolutely necessary to be believed by all Christians and cannot be unknown or denied without peril to salvation. In this sense, we now speak of fundamental articles.
V. Although all the truths revealed in Scripture are necessary to be believed as divine and infallible, yet they are not all equally necessary and the amplitude and extension of faith must be accurately distinguished here from its necessity. Not everything which belongs to the amplitude of faith must therefore belong to its necessity. All truths are not of the same weight. Some have a greater, others a lesser degree of necessity. For example, some are necessary by the necessity of means; others only by the necessity of precept. Some relate to doctrines strictly so called, others only to rites and ceremonies. One relates to some article or doctrine as to substance (for instance that Christ suffered and died); another to the same considered only with reference to circumstances (for instance, that Christ suffered under Pontius Pilate and was crucified between two thieves). For we might be ignorant of the latter without peril to our salvation.
Francis Turretin, Institutes of Elenctic Theology, ed. James T. Dennison Jr., trans. George Musgrave Giger, vol. 1 (Phillipsburg, NJ: P&R Publishing, 1992–97), 1.14.1–5 (p. 48–49).