The internal testimony of the Holy Spirit illuminates the believer so that he knows that the Scriptures are the Word of God. The biblical basis for this clarity is derived from two sources. First, the words of Scripture are self-attesting because they claim to be from God (2 Tim. 3:16; 2 Pet. 1:20–21). Second, the Holy Spirit’s dynamic power applies the truth of Scripture, resulting in a confident assurance in the Word itself (1 Cor. 2:4–16)… That does not mean that all who hear or read believe (Rom. 10:14–21), but it does mean that those who believe do so because of the convicting and illuminating work of the Holy Spirit.
John MacArthur