Related Topic: Sola Scriptura
Since for unbelieving men religion seems to stand by opinion alone, they, in order not to believe anything foolishly or lightly, both wish and demand rational proof that Moses and the prophets spoke divinely. But I reply: the testimony of the Spirit is more excellent than all reason. For as God alone is a fit witness of himself in his Word, so also the Word will not find acceptance in men's hearts before it is sealed by the inward testimony of the Spirit. The same Spirit, therefore, who has spoken through the mouths of the prophets must penetrate into our hearts to persuade us that they faithfully proclaimed what had been divinely commanded ...
John Calvin Institutes of the Christian Religion 1.7.5.
Compare Scripture with Scripture. False doctrines, like false witnesses, agree not among themselves.
The Scripture is to be its own interpreter, or rather the Spirit speaking in it; nothing can cut the diamond but the diamond; nothing can interpret Scripture but Scripture.
You all have by you a large treasure of divine knowledge, in that you have the Bible in your hands; therefore be not contented in possessing but little of this treasure. God hath spoken much to you in the Scripture; labor to understand as much of what he saith as you can. God hath made you all reasonable creatures; therefore let not the noble faculty of reason or understanding lie neglected. Content not yourselves with...divine truth...you accidentally gain in conversation; but let it be very much your business to search for it, and that with the same diligence and labor with which men are wont to dig in mines of silver and gold.
The Scripture is both the breeder and feeder of grace. How is the convert born, but by "the word of truth" (James 1:18)? How doth he grow, but by "the sincere milk of the Word" (1 Peter 2:2)?