"The grass withers and the flowers fall, but the word of our God endures forever." — Isaiah 40:8
What's your favorite "desert island" book? In other words, if you were stranded on a desert island, what one book would you want to have with you? Martin Luther would no doubt have answered, "The Bible."
Among the reasons for choosing the Bible as their most important book, Luther and the other Reformers would have certainly mentioned the Good Book's sufficiency: the Bible — and only the Bible — is enough.
The Bible's sufficiency is the final quality included in the Reformation idea of sola Scriptura ("Scripture alone"). In it, the Reformers declared that the Bible speaks on everything we need to understand for salvation and living God's way. Though it leaves us ignorant about astronomy (and many other aspects of the physical world), the Bible clearly reveals and declares that the earth, sun, moon, and stars were made by God and "declare the glory of God" (Psalm 19:1).
The Bible also helps us understand who we are. In short, the Bible tells us that because of our sin, we desperately need help. And the only One who can save us is Jesus Christ. Further, the Bible tells us how to live in obedience to God and how to love our neighbors and God's creation.
Hopefully you'll never need to choose a "desert island" book, but there is only one book sufficient to guide us into full life with God: his book. Read it. Meditate on it. Live by it.