In my lengthy critique of Sola Scriptura, Not by Scripture Alone: A Latter-day Saint Refutation of Sola Scriptura, I discuss, at the opening sections of the essay, how Protestant apologists admit that no biblical author could have practised the doctrine as they did not have the entirety of the Bible (for sola scriptura to be true, there first must be tota scriptura).
The following from Bernard Ramm, a Protestant who himself held to sola scriptura, further emphasises this point
Section 23: is there a testimonium without Scripture?
The doctrine of the testimonium must not be bound too dogmatically to the written truth of God. There were times when men had no written Word of God, and certainly the witness of the Spirit cannot be denied them. John Calvin, who sketched out the testimonium for the first time in the history of dogmatics, did not exclude the patriarchs from the testimonium (IV, viii, 5). Abraham had a real evangelical experience of such a nature that he is the pattern of faith for both Gentile and Jew (Rom. 4:11-12). He was justified by faith and without question was regenerated. Therefore he really participated in Jesus Christ. Can Abraham be denied the testimonium? There is no ground upon which this may be done. When the Lord of glory appeared to Abraham and called him to leave his homeland, the Spirit of God was working within his heart in such a way that Abraham perceived this call to be the voice of God. And when he believed God and was justified, and exhibited such unbounded faith in the presence of the impossible (Rom. 4:18-21), we cannot doubt that this was accompanied by an inner working of God’s Spirit.
When Paul preached to pagans who heard and believed even though they had no Old or New Testament, can it be maintained that they had no testimonium? They most assuredly did (John Lawsen writes that Irenaeus knew of unlettered tribes who had salvation written in their hearts by the Holy Spirit and were real Christians in the absence of a written record. The Biblical Theology of Saint Irenaeus, p. 24)! For the testimonium is given in connection with the revelation of God in whatever form it might happen to be. He is the Spirit of truth, and wherever God speaks his truth the Spirit seals it as God’s truth. We must gladly admit that in ages where there was no written Word or in conditions where the truth of God was yet in oral form (as in the preaching ministry of Christ and his apostles), or where the truth of God in Scripture is known mediately through sermon, song, or Christian literature, the Spirit yet bears his witness (Luke tells Theophilus that he is sending him a written life of the Savior to replace the oral teaching [katécheo, to be orally instructed] that he might have a certain record [Luke 1:1-4]. Theophilus, then, must have come to Christ and was instructed in Christian matters by means of oral instructions).
God does not leave us to ourselves in the surety of our salvation, but grants our salvation with a plenitude of certainty. This is the certainty of the testimonium, especially in the sense of bearing witness to our sonship, our divine adoption. And from this witnessing emerges our certainty. Whether we have the gospel orally or in print, we are heirs of the same certainty. Therefore, whether we know the truth by sermon, song, or printed page, the Spirit does work and does witness. Certainly there are primitive peoples today who know only the preached Word of God, yet who most assuredly have the testimonium within themselves. In our day, if a Christian has any means or ability, he must trace his certainty back to Sacred Scripture; and everything derived from Scripture must in turn be examined on the basis of Scripture. (Bernard Ramm, The Witness of the Spirit: An Essay on the Contemporary Relevance of the Internal Witness of the Holy Spirit [Grand Rapids, Mich.: Eerdmans, 1959], 98-99)
The section references from Calvin’s Institutes also stresses this point, something that is all too often ignored by modern Protestant defenders of sola scriptura:
But though the rule which always existed in the Church from the beginning, and ought to exist in the present day, is, that the servants of God are only to teach what they have learned from himself, yet, according to the variety of times, they have had different methods of learning. The mode which now exists differs very much from that of former times. First, if it is true, as Christ says, "Neither knoweth any man the Father save the Son, and he to whomsoever the Son will reveal him," (Mat 11: 27), then those who wish to attain to the knowledge of God behaved always to be directed by that eternal wisdom. For how could they have comprehended the mysteries of God in their mind, or declared them to others, unless by the teaching of him, to whom alone the secrets of the Father are known? The only way, therefore, by which in ancient times holy men knew God, was by beholding him in the Son as in a mirror. When I say this, I mean that God never manifested himself to men by any other means than by his Son, that is, his own only wisdom, light, and truth. From this fountain Adam, Noah, Abraham, Isaac, Jacob, and others, drew all the heavenly doctrine which they possessed. From the same fountain all the prophets also drew all the heavenly oracles which they published. For this wisdom did not always display itself in one manner. With the patriarchs he employed secret revelations, but, at the same time, in order to confirm these, had recourse to signs so as to make it impossible for them to doubt that it was God that spake to them. What the patriarchs received they handed down to posterity, for God had, in depositing it with them, bound them thus to propagate it, while their children and descendants knew by the inward teaching of God, that what they heard was of heaven and not of earth. (Institutes, IV.8.5)
