Here’s the summary of an important essay on this subject by Dr. Greg Bahnsen:
While the Bible teaches its own inerrancy, the inscripturation and copying of God’s Word require us to identify the specific and proper object of inerrancy as the text of the original autographa. This time-honored, common-sense view of evangelicals has been criticized and ridiculed since the days of the modernist controversy over Scripture. Nevertheless, according to the attitude of the biblical writers, who could and did distinguish copies from the autographa, copies of the Bible could serve the purposes of revelation and function with authority only because they were assumed to be tethered to the autographic text and its criteriological authority. The evangelical doctrine pertains to the autographic text, not the autographic codex, and maintains that present copies and translations are inerrant to the extent that they accurately reflect the biblical originals; thus the inspiration and inerrancy of present Bibles is not an all-or-nothing matter. Evangelicals maintain the doctrine of original inerrancy, not as an apologetic artifice, but on sound theological grounds: (1) the inspiration of copyists and the perfect transmission of Scripture have not been promised by God and (2) the extraordinary quality of God’s revealed Word must be guarded against arbitrary alteration. The importance of original inerrancy is not that God cannot accomplish His purpose except through a completely errorless text, but that without it we cannot consistently confess the veracity of God, be fully assured of the scriptural promise of salvation, or maintain the epistemological authority and theological axiom of sola Scriptura (for errors in the original, unlike those in transmission, would not be correctable in principle). We can be assured that we possess the Word of God in our present Bibles because of God’s providence; He does not allow His aims in revealing Himself to be frustrated. Indeed, the results of textual criticism confirm that we possess a biblical text that is substantially identical with the autographa. Finally, contrary to recent criticisms, the doctrine of original inerrancy (or inspiration) is not unprovable, is not undermined by the use of amanuenses by the biblical writers, and is not contravened by the New Testament use of the Septuagint as “Scripture.” Therefore, the evangelical restriction of inerrancy to the original autographa is warranted, important, and defensible; further, it does not jeopardize the adequacy and authority of our present Bibles. Accordingly, the doctrine of original inerrancy can be commended to all believers who are sensitive to the authority of the Bible as the very Word of God and who wish to propagate it as such today.