Because Scripture is the only source of special revelation that we possess, it is the only infallible and final authority for the church. This is the doctrine of sola Scriptura, which the Protestant Reformers embraced in order to correct errors that said other authorities such as post-biblical church tradition are equivalent to Scripture as a rule of faith. As we have seen, the doctrine of sola Scriptura is a necessary consequence of passages such as 2 Timothy 3:16–17, which identifies only Scripture as "God-breathed," or divinely inspired. And because God is incapable of erring, whatever He inspires is likewise incapable of teaching error.
Our doctrine of biblical infallibility, which says that the Bible cannot teach error, has to do with the capability of Scripture—what it can and cannot do. But if Scripture is unable to teach falsehood, that has implications for Scripture regarding what it actually is. Because the Bible is incapable of teaching error, Scripture is actually free from error. The Word of God does not affirm anything that is false, and we refer to this doctrine as the doctrine of Scriptural or biblical inerrancy.
Scriptural inerrancy is a good and necessary consequence of biblical infallibility, but it is also taught explicitly by the biblical writings. In today's passage, for example, we find a strong affirmation that every word of the Lord proves to be true (Ps. 18:30). Commenting on this text, John Calvin writes, "The word of God is pure, and without any mixture of fraud and deceit, like silver which is well refined and purified from all its dross." Other important passages that demonstrate the inerrancy of Scripture include John 17:17, wherein Jesus says to His Father, "Your word is truth."
Importantly, when we speak of biblical inerrancy, we are speaking of the original text of Scripture, not its manuscript copies. We do not possess the actual hard copies that the Apostles and prophets wrote; instead, we have copies of these writings. Since only the Apostles and prophets were inspired, only the text that they wrote is inerrant. Various copies may, and do, contain differences, additional words, and other discrepancies between them. This is not a problem, however, for the Bible is preserved better than any other ancient book, and we are able to reconstruct the original text that the Apostles and prophets wrote even though all we possess is many, many manuscripts from scribes who copied the Bible.
We do not have the original manuscript copies that the Apostles and prophets wrote; however, we can determine what the original text is that they wrote by comparing the various manuscripts that we do have. We can be confident, then, that we have an inerrant Bible in its original languages. We need not fear that the Scriptures have any errors, so we may fully trust these writings. In so doing, we are trusting God Himself.