I have found many opponents of Protestantism—along, I’m afraid, with some Protestants themselves—to be strangely confused on this question. But the distinction is really pretty basic:
Sola Scriptura: the Bible is the only “rulebook” that sets out Christian doctrine. Although we don’t need anything else to know God and be saved by him, God didn’t deliver it in a hermetically-sealed chamber; rather, he put it in the middle of a vast library full of other sources that help us understand it better.
Solo Scriptura: the Bible is the only “rulebook” that sets out Christian doctrine, and any attempt to nuance or finesse our understanding of it using other sources of knowledge is probably from the devil. We don’t need that malarky, we don’t want that malarky, and in fact, you should all probably split up and each go into a separate forest with your Authorized Version and read it alone for the rest of your lives like hermits.
As you can see, Sola Scriptura and Solo Scriptura could agree on claims such as:
- The Bible, when read carefully in plain English, is sufficient in clarity and content to bring anyone to a saving knowledge of God.
- Historical creeds, confessions, and generally-agreed doctrines are not necessarily correct, and a Christian doesn’t need to know them.
However, Sola Scriptura would start to diverge quite rapidly from Solo Scriptura, by affirming claims such as:
- Historical creeds, confessions, and generally-agreed doctrines are useful for staking out the prima facie rational bounds of orthodoxy. They are not inerrant, but when we find ourselves going against or beyond them, we should take extra care to ensure our conclusions are accurate.
- Contemporary sources, such as the Ugaritic archives, Second Temple literature, and Roman historiography, are of great value in helping us contextualize and add detail to what is written in the Bible. (Ironically, Solo Scriptura, in denying this, actually ends up denying the value of studying the original languages of the Bible to better understand its meaning.)
- Where Scripture appears ambiguous on some issue, God does not require us to know the answer with certainty, but it is nonetheless good to consult available contemporary sources that can clarify what God intended to convey.
- The private interpretation of the individual is only as good as his exegetical skill and historico-grammatical knowledge.
Solo Scriptura, on the other hand, seems to be characterized by attitudes such as:
- The Bible is so completely clear on everything it teaches that we need not (and perhaps even cannot) augment or refine our understanding of it using external contemporary sources.
- To appeal to external evidence is to deny the Bible’s sufficiency and perspicuity.
- There’s therefore really no need to consult commentaries, examine exegetical arguments, refer to contemporary sources, or understand socio-linguistic context. (In fairness, not everyone will say this; but I have met people who really believe that this the Protestant approach to Scripture.)
Obviously these claims are just silly. The Bible is sufficient for doctrine and practice, but that doesn’t mean we cannot refine or expand our understanding of it using other sources. To say this is really to deny our obligation to understand God’s word using all the resources he has made available, and even to deny that he has providentially arranged these resources for us. It ends up saying that the Bible is the only source of truth—a self-refuting statement if ever there were one, since we must first know truths about language to read it.
Sola Scriptura, on the other hand, says that the Bible is the only inerrant source of truth, the only God-breathed source of truth, but certainly not the only source period.