First, it is necessary to understand what sola Scriptura does and does not assert. The Reformation principle of sola Scriptura has to do with the sufficiency of Scripture as our supreme authority in all spiritual matters. Sola Scriptura simply means that all truth necessary for our salvation and spiritual life is taught either explicitly or implicitly in Scripture.

It is not a claim that all truth of every kind is found in Scripture. The most ardent defender of sola Scriptura will concede, for example, that Scripture has little or nothing to say about DNA structures, microbiology, the rules of Chinese grammar, or rocket science. 

This or that “scientific truth” for example, may or may not be actually true, whether or not it can be supported by Scripture—but Scripture is a “more sure Word,” standing above all other truth in its authority and certainty. It is “more sure,” according to the apostle Peter, than the data we gather firsthand through our own senses (2 Pet. 1:19). Therefore Scripture is the highest and supreme authority on any matter to which it speaks. But there are many important questions on which Scripture is silent. Sola Scriptura makes no claim to the contrary.

Nor does sola Scriptura claim that everything Jesus or the apostles ever taught is preserved in Scripture.  It only means that everything necessary, everything binding on our consciences, and everything God requires of us is given to us in Scripture.

Furthermore, we are forbidden to add to or take way from Scripture (cf. Deut. 4:2; 12:32, cf. Rev. 22:18-19). To do so is to lay on people’s shoulders a burden that God Himself does not intend for them to bear (cf. Matt. 23:4).

Scripture is therefore the perfect and only standard of spiritual truth, revealing infallibly all that we must believe in order to be saved, and all that we must do in order to glorify God. That—no more, no less—is what sola Scriptura means.

The Westminster Confession of Faith defines the sufficiency of Scripture like this:

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 (1:6).

The Thirty-nine Articles of the Anglican Church include this statement on sola Scriptura:

Holy Scripture containeth all things necessary to salvation: so that whatsoever is not read therein, nor may be proved thereby, is not to be required of any man, that it should be believed as an article of the Faith, or be thought requisite or necessary to salvation (article 6).

So sola Scriptura simply means that Scripture is sufficient. The fact that Jesus did and taught many things not recorded in Scripture (Jn. 20:30; 21:25) is wholly irrelevant to the principle of sola Scriptura. The fact that most of the apostles’ actual sermons in the early churches were not written down and preserved for us does not diminish the truth of biblical sufficiency one bit. What is certain is that all that is necessary is in Scripture—and we are forbidden “to exceed what is written” (1 Cor. 4:6).

Scripture clearly claims for itself this sufficiency—and nowhere more clearly that 2 Timothy 3:15-17. A brief summary of that passage is perhaps appropriate here as well. In short, verse 15 affirms that Scripture is sufficient for salvation: “The sacred writings . . . are able to give you the wisdom that leads to salvation through faith which is in Christ Jesus.” Verse 16 affirms the absolute authority of Scripture, which is “God-breathed” (Gk. theopneustos) and profitable for our instruction. And verse 17 states that Scripture is able to equip the man of God “for every good work.” 

So the assertion that the Bible itself does not teach sola Scriptura is simply wrong. (Scripture, Tradition, and Rome, Part 2)

John MacArthur