Whence Scripture’s authority becomes known to us.
§ 20. You might say the question is Whence is Scripture’s authority made know to us? We respond. Those who are within the church do not question Scripture’s authority, for it is the principle. How could they truly be disciples of Christ if they wanted to call into doubt Christ’s teaching? How could they be true members of the church if they wanted to doubt the church’s foundation? How will they demand that that be proved which they always assume to prove other things? How could they doubt it when they sense its efficacy in their hearts? “The Holy Spirit testifies” in their hearts “that He is the Spirit of truth,” that is, that the doctrine that has come from the Holy Spirit is unchanging truth (1 John 5:6). If in times of temptation they begin to doubt Scripture’s authority, they must deal with those [doubts] as they would with deniers, for doubt is next to denial.
§ 21. Those outside the bounds of the church, who forcefully and recalcitrantly resist the truth, cannot be persuaded by any argumentation. With those who may be helped, the situation for us will be that, if they only partially reject Scripture, they can be convinced from the other part. Thus the Jews can be convinced of the truth and authority of the New Testament based on the Old, which they accept.
§ 22. However, if those who totally reject [Scripture] are to be persuaded of its authority, then one should have recourse to the three classes [of witnesses] given above. For though the Holy Spirit is the chief and sup0reme witness, this is not where one should start in the conversion of such people; that is to say, they should not be told just to wait until the Holy Spirit, apart from means, testifies of Scripture’s authority in their hearts. Rather, they should be brought to the witness of the church, which in this respect serves the role of a teacher for the pagan student. Therefore just as “a student must first suspend disbelief” until he can later come to a decision for himself about what he has been taught, so also must the pagan give his assent to the church’s witness, this being the first stage of inquiry into Scripture’s authority. Then one should add the internal proofs of antiquity, the prophecies, the miracles, etc., seeing as the sole witness of the church alone is insufficient for persuading the pagan of Scripture’s divine authority since he may still doubt whether that church is the true church of God. Therefore just as a teacher must not only set forth his teachings but also must corroborate their veracity, so also it is insufficient for the church to assert “These are the divine Scriptures” without also offering the reasons for its assertion. Then at last it may follow that the Holy Spirit will Himself bear witness in [the pagan’s] heart and seal the truth of His own words. On this Augustine (Contra epist. Fundam., ch. 5) writes thus: “Let us follow those who first invite us to believe what we cannot yet perceive so that, strengthened in our faith, we may come to understand what we believe, not anymore because of men but because of God Himself internally confirming and enlightening our minds.”
Gerhard, Johann On Interpreting Sacred Scripture and Method of Theological Study 11-12