What Does “Scripture Alone” Mean, and Why Should You Care?
When we talk about sola scriptura, we are talking about the fact that it is God’s word—not man’s—that gives us the instruction we need to attain everlasting life. It’s not to say that Christians should only read the Bible and nothing else. If your sink gets clogged, a plumbing manual will be of more use than anything in the Old or New Testament. Sola scriptura means that the Bible gives us everything we need to know about everything that truly matters—specifically, our salvation.
The Sufficiency of Scripture
At the time of the Reformation in the sixteenth century, the Roman Catholic Church did not deny the importance of the Word of God but rather its sufficiency. Rome said Scripture was insufficient to reveal to us the way to heaven. Rather, Rome argued that we needed something in addition to Scripture: the traditions of the church. According to theologian Michael Horton in The Christian Faith,
The Council of Trent [in the sixteenth century] established the view that Scripture and tradition are actually two forms of God’s Word—”written” and “unwritten".” (p. 188)
What led the Roman Catholics astray was their understanding that the church birthed the Word of God, rather than the Word being the foundation of the church (Eph. 2:20). Yes, God gives us consciences, good sense, and even the traditions of the church from which we can glean insight into life, but knowledge of salvation is found in Scripture and Scripture alone.
How could we even for a moment think we could bring any kind of insight of our own to add to God’s word? In his prophecy, Isaiah speaks of God’s word not returning “empty” but accomplishing God’s purpose (Isa. 55:11). Proverbs 30:6 describes every word of God as true, and moreover, gives a warning to any who would dare add their own words to it. Similar scriptural curses in Deuteronomy 4:2 and 12:32 and Revelation 22:18-19 show just how seriously God takes the sufficiency of his word.
In Paul’s second letter to Timothy, he writes:
All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work. (2 Tim. 3:16-17)
This is Scripture’s greatest defense for its own sufficiency—the fact that the Bible is God’s Word breathed out by Him, and not man’s.
This doctrine of the nature of Scripture continued from the time of the Bible’s formation through the age of the ancient church. The church’s high view of Scripture is what the Reformation was trying to recover. In his work Against Heresies, the great apologist Irenaeus said, “The Scriptures are perfect, inasmuch as they were uttered by the Word of God and His Spirit.” Augustine wrote, “Therefore we yield to and agree to the authority of the Holy Scripture which can neither be deceived nor deceive.” 
The Supreme Authority of Scripture
Thus, the creeds and confessions of the Reformation spoke clearly about the necessity of recovering the sufficiency and supreme authority of Scripture. Two are worth quoting at length. First, the Geneva Confession sets forth in Section One:
We affirm that we desire to follow Scripture alone as a rule of faith and religion, without mixing with it any other things which might be devised by the opinion of men apart from the Word of God, and without wishing to accept for our spiritual government any other doctrine than what is conveyed to us by the same Word without addition or diminution, according to the command of our Lord.
The French Confession of Faith from 1559 is equally clear:
We believe that the Word contained in these books has proceeded from God, and receives its authority from Him alone, and not from men. And inasmuch as it is the rule of all truth, containing all that is necessary for the service of God and for our salvation, it is not lawful for me, nor even for angels, to add to it, to take away from it, or to change it.
This was the thrust of the Reformation: turning aside from man and relying fully upon the Word of God. It was a rejection of the notion that the church had anything to offer toward salvation apart from ministering what the Bible taught. This is why our worship services are filled with Scripture: we sing Scripture, pray Scripture, read Scripture, and preach Scripture, because it alone is sufficient for our salvation.
Knowing Scripture by R. C. Sproul
 Augustine, A Treatise on the Merits and Forgiveness of Sins, and on the Baptism of Infants, Book I, Ch. 33.
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