Theology must be Scripturally grounded. God’s life-giving speech reveals to us His salvation and calls us to faith and repentance. We were once darkness, but now we are light in the Lord (Eph. 5:8). Satan blinds the minds of unbelievers, lest they see the light of the gospel of the glory of Christ (2 Cor. 4:4). Yet the God who commands light to shine out of darkness shines in our hearts, giving us the light “of the glory of God in the face of Jesus Christ” (2 Cor. 4:6). God always does this by the Spirit working through the Word. The Holy Spirit is the Author of Scripture, and He speaks through Scripture (Heb. 3:7). The Scriptures teach us everything that we need to make us wise for salvation through faith in Christ and to be thoroughly equipped for every good work (2 Tim. 3:15–17). This is why those who do not speak according to the rule of Scripture have no light in them (Isa. 8:20). Yet without the Spirit, even the Scriptures cannot help us. We are dead in trespasses and sins (Eph. 2:1–2), our minds and hearts are darkened (Eph. 4:18; Rom. 1:21), and we need someone to raise us from the dead and turn on the lights (Eph. 5:14). If Scripture is sufficient to make us wise for faith and life in Christ, then Scripture alone can be our guide to walking with God. Everything else is both useless and superfluous. Yet we must be born of the Spirit in order to see God’s kingdom (John 3:5). Only through the Spirit working by and with the Scriptures in our hearts can we walk in the light with the God who is light (1 John 1:7).
The Reformation principle of sola Scriptura has to do with the sufficiency of Scripture as our supreme authority in all spiritual matters. Sola Scriptura simply means that all truth necessary for our salvation and spiritual life is taught either explicitly or implicitly in Scripture. It is not a claim that all truth of every kind is found in Scripture. The most ardent defender of sola Scriptura will concede, for example, that Scripture has little or nothing to say about DNA structures, microbiology, the rules of Chinese grammar, or rocket science. This or that “scientific truth,” for example, may or may not be actually true, whether or not it can be supported by Scripture—but Scripture is a “more sure Word,” standing above all other truth in its authority and certainty. It is “more sure,” according to the Apostle Peter, than the data we gather firsthand through our senses (2 Peter 1:19). Therefore, Scripture is the highest and supreme authority on any matter on which it speaks.
But there are many important questions on which Scripture is silent. Sola Scriptura makes no claim to the contrary. Nor does sola Scriptura claim that everything Jesus or the Apostles ever taught is preserved in Scripture. It only means that everything necessary, everything binding on our consciences, and everything God requires of us is given to us in Scripture (2 Peter 1:3).
Scripture is the perfect and only standard of spiritual truth.
Furthermore, we are forbidden to add to or take away from Scripture (cf. Deut. 4:2; 12:32; Rev. 22:18-19). To add to it is to lay on people a burden that God Himself does not intend for them to bear (cf. Matt. 23:4).
Scripture is therefore the perfect and only standard of spiritual truth, revealing infallibly all that we must believe in order to be saved and all that we must do in order to glorify God. That—no more, no less—is what sola Scriptura means.
“The whole counsel of God, concerning all things necessary for his own glory, man’s salvation, faith, and life, is either expressly set down in scripture, or by good and necessary consequence may be deduced from scripture: unto which nothing at any time is to be added, whether by new revelations of the Spirit, or traditions of men.” —Westminster Confession of Faith