Pythagoras is said to have been the earliest outside of Scripture (Isa. 40:22) to contend that the earth is round. He did not make the earth round with his assertions, but identified what already was. Sir Isaac Newton certainly did not create gravity, but he is credited for our modern understanding of it. Likewise, the term “sola scriptura” is not found in scripture (similar to terms like “trinity” and “omniscience”), but it was coined during the “Reformation Movement” as part of Martin Luther’s protests against perceived corruptions of the Catholic Church. It was a “Latin phrase (literally ‘by Scripture alone’) describing the Protestant theological principle that Scripture is the final norm in all judgments of faith and practice. Church traditions and customs, pronouncements of church officials, civil law or any other purely human source, including human reason, must yield to clear scriptural pronouncements” (Reid, Daniel G., et al. Dictionary of Christianity in America, 1990: n.p.). Did the Protestant Reformers, who, incidentally, got unfortunately got so many things wrong, originate that idea? Because they were wrong on many doctrinal conclusions, does that automatically make the idea of “sola Scriptura” incorrect? It seems to me that at least three questions are in order regarding this subject.
What does “by Scripture alone” mean?
It means that the Bible does not share authority with anyone or anything else. One author says it meant “’the freedom of Scripture to rule as God’s word in the church, disentangled from papal and ecclesiastical magisterium and tradition.’ It viewed the Word as supreme over tradition and the sacraments” (McArthur, John. Expository Preaching, 1992. Dallas: Word Pub., 47). A creed book, discipline, or annual church conference may vote and decide about what a religious group’s view on a matter should be. They may even change their view from a previously held, correct view. Or, a religious group may claim to have received latter day revelation and may produce a book they claim to be co-authoritative with the Bible. Or, they may say “the church” and “church tradition” is co-authoritative. The idea of “by Scripture alone” rejects competing or co-authoritative standards.
It does not eliminate the need to “handle aright” or involve hermeneutics (the science of interpretation). That is a cognitive necessity. You cannot read even the simplest of instructions or follow the most basic of tasks without employing logic, reason, and deduction. That is not the same thing as a person, group, or book that claims to rival or co-authorize with Scripture.
What is the alternative?
That question has really already been answered. The alternative is to suggest that Scripture alone is insufficient or inadequate, that is not the sole authority on matters of truth and right. Some would even call the idea of following only Scripture as destructive heresy. Yet, the alternative to Scripture alone is Scripture along with something else, whether a man, group, council, church, or governing body.
Why is it so important?
This is the crux of the matter. Scripture is God-breathed, making one spiritually complete (2 Tim. 3:16-17). If Scripture is sufficient, what need is there for anything beyond it? On what basis would we accept anything more or less than or different from the Bible? How could fallible man be equal to or co-authorize with the perfect law of the Lord? Let us accept no substitute or rival to the Bible!
The reader is encouraged to consider some excellent thoughts on this subject from here: https://www.christiancourier.com/articles/557-what-is-sola-scriptura