Hebrews 4:12

After we die, or experience the Rapture, God will do a spiritual postmortem on us at the judgment seat of Christ (Romans 14:10-12; 2 Corinthians 5:10). He will examine our innermost attitudes and motives. The "scalpel" He will use is His Word. The Word of God is "living" because it is the word of the living God (Hebrews 3:12), and it is "active" (energetic, powerful). The sword in view (Gr. machairan) was originally a small one like a boning knife that cooks used to cut up meat. In its double-edged form it was a symbol of judges and magistrates in the Roman world. It illustrated the power of those officials to turn both ways to get to the bottom of a case. However it is possible that by the time Hebrews was written machaira (sword) had come to mean a sword of any size, long or short. [Note: Moffatt, p. 56.] The Word of God can express and distinguish what is "soulish" (natural) and what is spiritual in our motivation and actions. It can do so even when those elements are as close to each other as our joints and marrow. It is even able to expose our thoughts and attitudes (cf. 1 Corinthians 4:5).

"What the author is saying is that God’s Word can reach to the innermost recesses of our being. We must not think that we can bluff our way out of anything, for there are no secrets hidden from God. We cannot keep our thoughts to ourselves." [Note: Morris, p. 44.]

Many Christians use this verse to show that God will judge unbelievers with His piercing Word, but in the context it refers to God judging believers to determine rewards (1 Corinthians 3:11-15).