I'm steadily working my way through Reformed Dogmatics Volume 2: God and Creation, by Herman Bavinck, Baker Academic, 2006. In his treatment of the doctrine of the Trinity the theologian defends the use of extrabiblical terminology in clarifying and explaining the teaching of the Bible. The trouble is that false teachers cite Scripture as well as the orthodox, so issues won't be settled simply by both sides trading proof texts. When the friendly neighbourhood "Jehovah's Witnesses" turns up on your doorstep you may tell them, "Ah, but we believe that Jesus is the Son of God." But they will respond, "So do we as a matter of fact." Then it comes down to what is meant by "Jesus is the Son of God". Is he the greatest creature that God ever made, or is he fully God, equal to the Father in his divine being and glory? The same problem was faced by the early church in responding to the challenge of Arianism. Hence the Nicene Creed's statement that the Son is homoousios - of the same essence as the Father. The Church was forced to resort to extra-biblical terminology in order to defend the divine identity of Jesus Christ.
It has often been the case that false teachers have resisted the use of non-biblical language in defining what the Bible says. Arians, Socinians and "Jehovah's Witnesses" tend to be strict biblicists. They protest that they base their teaching solely on the words of Scripture. However, therein lies their subterfuge. They undermine biblical truth by emptying Scripture expressions of their true meaning. They will happily say that Jesus is the Son of God, but they deny that as the Son of God he is of the same essence as the Father.
Bavinck makes it clear that the church's use of extrabiblical terms does not involve the introduction of newly minted extrabiblical teaching. Rather such language is needed to defend the truth against all error. He makes the interesting point,
Under the guise of being scriptural, biblical theology has always strayed farther away from Scripture, while ecclesiastical orthodoxy, with its extrabiblical terminology, has been consistently vindicated as scriptural. (p. 297).
So, the church needed to resort to other than biblical language in order to safeguard the integrity of biblical truth. But, more positively, Bavinck insists that use of extrabiblical language is essential to the church's constructive theological task,
Scripture after all has not been given us simply, parrotlike to repeat it, but to process it in our own minds and to reproduce it in our own words. Jesus and the apostles used it in that way. They not only quoted Scripture verbatim, but also by a process of reasoning drew inferences from it. Scripture is neither a book of statutes for a dogmatic textbook but the foundational source of theology. As the Word of God, not only its exact words but also the inferences legitimately drawn from it have binding authority. Furthermore, reflection on the truth of Scripture and the theological activity related to it is in no way possible without the use of extrabiblical terminology... Involved in the use of such terms, therefore is the Christian's right of independent reflection and theology's right to exist. (p. 296)
What Bavinck is saying here is in accordance with the principle set out in the Westminster Confession of Faith,
The whole counsel of God concerning all things necessary for His own glory, man's salvation, faith and life, is either expressly set down in Scripture, or by good and necessary consequence may be deduced from Scripture (I:VI) [Emphasis added].
Theology is an act of holy reason. It involves faith seeking understanding by thoughtfully reflecting on God's self-revelation in Holy Scripture. Bavinck gives further attention to this matter Reformed Dogmatics Volume 1: Prolegomena, Baker Academic 2003, p. 617ff. Without recognition of this point not only theology, but also preaching would prove impossible. For what is preaching but an attempt to explain and apply the teaching of the Bible? We cannot do that simply by quoting a string of Bible texts, one after another. The preacher has to elucidate the meaning of his text by using extrabiblical language. He has to work out how his text relates to the teaching of other portions of Scripture. He must reflect on how believers should live in the light of the passage he is expounding. These "good and necessary consequences deduced from Scripture" are part and parcel of the whole counsel of God to which the whole people of God need to be exposed. That is one of the reasons why "the preaching of the Word of God is the Word of God." (Second Helvetic Confession Chapter I.) Thus the preacher's task entails 'holy reasoning' akin to the efforts demanded of the theologian. Indeed, preaching is nothing less than 'theology on fire'.
Bavinck words on the relationship between faith and reason have resonance for would-be pastor-theologians,
Believing is the natural breath of the children of God. Their submission to the Word of God is not slavery but freedom. In that sense faith is not a sacrifice of the intellect but mental health (sanitas mentis). Faith, therefore, does not relive the Christian of the desire to study and reflect; rather it spurs them on. Nature is not destroyed by regeneration but restored. (RD Vol 1, p. 616-617).