“For the word of God is living and powerful, and sharper than any two-edged sword, piercing even to the division of soul and spirit, and of joints and marrow, and is a discerner of the thoughts and intents of the heart. And there is no creature hidden from His sight, but all things are naked and open to the eyes of Him to whom we must give account” (Heb. 4:12,13) NKJV).
Believer’s Bible Commentary observes that “[These] two verses contain a solemn warning that unbelief never goes undetected. It is detected first by the word of God. (The term used here for word is logos, the familiar word used by John in the prologue to his Gospel. However, this verse refers, not to the Living Word, Jesus, but to the written word, the Bible.) This word of God is:
- living—constantly and actively alive.
- cutting—sharper than any two-edged sword.
- dividing—piercing the soul and spirit, the two invisible, nonmaterial parts of man. Piercing the joints and marrow, the joints permitting the outward movements and the marrow being the hidden but vital life of the bones.
- discerning—discriminating and judging with regard to the thoughts and intents of the heart. It is the word that judges us, not we who judge the word.”
 Believer’s Bible Commentary by William MacDonald (red and bold font added)