One basic tenet of the Reformed doctrine of Scripture is that doctrine should be derived from Scripture as its source, but that Scripture “knows” more than it puts forward explicitly in so many words. That is, the words of Scripture themselves necessitate further deductions that in turn provide matter for dogmatic reflection and formulation.
A very concise statement of this idea is found in Westminster Confession of Faith 1.6:
VI. 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: unto which nothing at any time is to be added, whether by new revelations of the Spirit, or traditions of men. Nevertheless, we acknowledge the inward illumination of the Spirit of God to be necessary for the saving understanding of such things as are revealed in the Word: and that there are some circumstances concerning the worship of God, and government of the Church, common to human actions and societies, which are to be ordered by the light of nature, and Christian prudence, according to the general rules of the Word, which are always to be observed.
One finds Gregory of Nazianzus expressing a similar idea in his Fifth Theological Oration, “On the Holy Spirit”:
XXIV. Since, then, there is so much difference in terms and things, why are you such a slave to the letter, and a partisan of the Jewish wisdom, and a follower of syllables at the expense of facts? But if, when you said twice five or twice seven, I concluded from your words that you meant Ten or Fourteen; or if, when you spoke of a rational and mortal animal, that you meant Man, should you think me to be talking nonsense? Surely not, because I should be merely repeating your own meaning; for words do not belong more to the speaker of them than to him who called them forth. As, then, in this case, I should have been looking, not so much at the terms used, as at the thoughts they were meant to convey; so neither, if I found something else either not at all or not clearly expressed in the Words of Scripture to be included in the meaning, should I avoid giving it utterance, out of fear of your sophistical trick about terms. In this way, then, we shall hold our own against the semi-orthodox—among whom I may not count you. For since you deny the Titles of the Son, which are so many and so clear, it is quite evident that even if you learnt a great many more and clearer ones you would not be moved to reverence. (Oration 31.24)