So then this Scripture alone is the fundamental principle by which, and the material from which, every saving truth must be drawn. It is the rule and standard by which every true and also very false teaching about the things of God must be determined (Isa 8:20; Luke 16:29; Acts 17:10, 11). In sum, it is a self-convincing and irrefutable witness and Judge, namely by virtue of its own demonstrated proof by which every controversy which arises over divine matters must be judged (John 5:39, 45).

Moreover, the criterion or norm for judging is comprised of these axioms: 1. Whatever Scripture contains, or whatever agrees with it either explicitly or by a necessary consequence from it, is true doctrine. 2. That which disagrees with it must be false. 3. Whatever is not contained in it, although it may not be openly opposed to it, is a teaching not necessary for salvation.

Synopsis of a Purer Theology, Disputation 3 Paragraphs 18 & 19.