"...and the sword of the Spirit, which is the word of God..." (Ephesians 6:17 ESV)
"And the Spirit is the one who testifies, because the Spirit is the truth." (1 John 5:6 ESV)
Gresham Machen writes,
If we take the Bible as the Word of God, then the Bible becomes our standard of truth and of life. When we are asked whether we can support any kind of message or can engage in any course of conduct, what we do is simply to compare that message or that course of conduct with the Bible. If it agrees with the Bible, we can support it or follow it; if it does not agree with the Bible, we cannot support it or follow it no matter what we may be told by other authorities to do. ...
Our standard is not a flexible standard. Far from holding that what is true today becomes false tomorrow according to the shifting needs of human life, we find our standard both of truth and of conduct in the Bible, which we hold to be not a product of human experience but the Word of God. ...
The "witness of the Spirit" is not, as it is often quite falsely represented today, independent of the Bible; on the contrary it is a witness by the Holy Spirit, who is the author of the Bible, to the fact that the Bible is true.
-- J. Gresham Machen, from Selected Writings