What is Scripture? The typical answer is that Scripture is the inspired Word of God. It is theopneustos, God-breathed. This is generally taken to mean that Scripture is the normative criterion for theology. When we think theologically we measure our theological judgments and have those judgments guided and rooted in Scripture. The word theopneustos is derived from 2 Timothy 3:16 and 17,
“All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness, so that the servant of God may be thoroughly equipped for every good work.”
But more often than not, when this passage is taken to support the authority or normativity of Scripture, it will be replied that Paul only had the Old Testament Scriptures in mind, if that. As such we can’t take it as a proof text for Scriptural inspiration.
In response to this objection I think it important to make a distinction which will help us to parse how this passage can be used. If the passage is taken to be a way of establishing what should be included in the canon, then this is clearly a mistake. If, as seems obvious, Paul was referring to the OT Scriptures then at most this passage will only serve to establish the canonicity of certain OT texts. However, when one talks about the inspiration or normativity of Scripture one isn’t making claims about the texts that should be included in the canon, but what kind of texts canonical texts are. In other words, whether Paul was referring to OT texts, NT texts, or whatever, his key point is to establish the authority of Scripture, whatever that Scripture contains.
To put it another way, the statement that Paul makes concerning Scripture is a ‘universally quantified statement’. William Lane Craig writes that ‘universally quantified statements’, “are true with respect to all the members of the domain of quantification7, existentially quantified statements are true with respect to some of the members of the domain of quantification.” (48, God Over All). When Paul writes that ‘all Scripture is God-breathed’ this is a universally quantified rather than an existentially quantified statement. Paul is referring to all the members of the class ‘Scripture’ whatever that class contains.
For example, if we ask the question ‘Is the gospel of John Scripture?’ And we reply in the affirmative, then the universally quantified statement, ‘all Scripture is God-breathed’ will apply to the gospel of John whether or not Paul had the gospel of John in mind. In which case I think that we can be confident that 2 Timothy 3:16 gives us ample warrant for believing in the authority of Scripture whether or not it tells us what can be included in the canon. For more on how to deduce what should be included in the canon B. B. Warfield has written helpfully and extensively. You can find one of his articles here: http://www.reformed.org/master/index.html?mainframe=/bible/warfield_canon.html