God's primary method of sanctification is His Word. Jesus said in His high priestly prayer, "Sanctify them in the truth; your word is truth" (John 17:17). We have a further elaboration on this point, I believe, in today's famous passage on the Bible. Here, Paul affirms two things about the Bible: the nature of the Bible (v. 15) and the purpose of the Bible (vv. 16-17). We will spend a few moments on the first point and in the rest of the message address the second point since that deals more directly with our sanctification.
The first point Paul makes is about the nature of the Bible: "All Scripture is breathed out by God…" (v. 16). I just want to observe two things briefly: 1) the Bible is the very Word of God as it is breathed out by God; 2) all Scripture is the Word of God, both the Old and New Testaments. Some argue that Paul could not have meant the New Testament because it was not complete at that time. Is that necessarily true? For one, Peter regarded Paul's letters on par with the Scriptures: "There are some things in [all Paul's letters] that are hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other Scriptures" (2 Pet. 3:16). Paul, too, recognized the scriptural authority of his own letters when he identified Himself as "an apostle of Christ Jesus by the will of God" (Eph. 1:1). An apostle was like today's special presidential envoys, who are sent out to conduct various tasks and businesses in the name and authority of the President. For Paul to write as an apostle of Christ was to write with the very authority of Christ. So, Paul's statement in today's passage applies not only to the Old Testament but also to the New Testament.