In his little booklet Can I Trust the Bible? R.C. Sproul discusses the terms “infallibility” and “inerrancy.” I appreciate the way he describes the difference and the need to maintain both:
The church historically has seen that the Bible alone, of all the written literature in history, is uniquely infallible. The word infallible may be defined as “that which cannot fail”; it means something is incapable of making a mistake. From a linguistic standpoint, the term infallible is higher than the term inerrant. Though the words have often been used virtually as synonyms in the English language, there remains a historic technical definition between the two. The distinction is that of the potential and the actual, the hypothetical and the real. Infallibility has to do with the question of ability or potential; that which is infallible is said to be unable to make mistakes or to err. By contrast, that which is inerrant is that which, in fact, does not err. As an illustration: a student can take a test made up of twenty questions and get twenty correct answers, giving him an inerrant test. However, the student’s inerrancy in this restricted arena does not make him infallible, as mistakes on subsequent tests would verify. (pp.26-27)
This is a good illustration of what medieval theologian John Duns Scotus called a formal distinction. Infallibility and inerrancy are both characteristics of Scripture. They can be distinguished, as Sproul did above, but they cannot be separated. They belong together.