I was recently in a conversation with a member of a fundamentalist Baptist church here in Cork, and one of the issues we discussed was Sola Scriptura, which is both the formal doctrine of Protestantism as well as its weak theological underbelly. In the course of our discussion, he stated that sola scriptura was operative during the time of Jesus and the New Testament Church, and was not a doctrine that came about after the completion of the New Testament. Such is not the historical view of sola scriptura, to be sure—more informed Protestants will state that sola scriptura can only be operative when tota scriptura is in existences—that is, the doctrine can only be put into practice once the totality of scripture has been inscripturated—it is not practiced during a time of inscripturation.
That this (weaker) form of sola scriptura is unbiblical can be seen in many places, not the least is that, during the time of inscripturation of the New Testament, the New Testament authors privileged oral traditions as being just as authoritative as the written Word of God; note the following examples:
Not I praise you, brethren, that ye remember me in all things, and keep the ordinances, as I delivered them to you. (1 Cor 11:2)
Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle. (2 Thess 2:15; cf. 1 Thess 2:13)
Now we command you, brethren, in the name of the Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition he received of us. (2 Thess 3:6).
The term translated as "tradition" (or, in the case of 1 Cor 11:2, "ordinances") is παραδοσις, which refers to something that is passed onto another. Such teachings include, for example, Paul's teachings on the Eucharist which was part of oral tradition, as we learn in 1 Cor 11:23f and the identity of the Messiah from the Old Testament (Acts 17).
Furthermore, the entirety of Scripture explicitly teaches the need for an active interpreter of God's word. In Num 15:32-36, we read a story of a man caught picking up sticks on the Sabbath. Although the people were aware of the commandment to keep the Sabbath day holy (Exo 20:8-10; 31:14-15, etc.), they did not know whether this general law applied to this specific situation. What did they do? Did they dispute over the written records they had in their possession and from different denominations, with each one emphasising certain texts over other texts, as one finds in much of Protestantism (compare the debates between Reformed and free-will Baptists, for instance)? No, instead, God, through his divinely appointed spokesman, Moses, gave an explicit revelation on this issue, that that "fine-tuned," if you will, the pre-existing revelation on this matter, namely that it was illegal to pick up sticks on the Sabbath, resulting in this man being stoned.
This is what an authoritative source outside of Scripture does--it "fine tunes" the information contained in Scripture, as well as give authoritative guidance on issues Scripture is silent on (modern examples within the realm of moral theology would include homosexual adoption; transsexual issues; abortion; pornography; test-tube fertilisation; euthanasia).
