What does Hebrews 4:13 mean?

The Bible often points out the futility of hiding from God. Adam and Eve attempted to hide from God after they sinned in Eden (Genesis 3:8–10). Cain tried to hide his murder of Abel from God (Genesis 4:9). Jonah hid on a boat (Jonah 1:1–4). Everything we think is hidden is crystal clear to God, including our own thoughts and motivations. We may fool other people (1 Samuel 16:7), and we may even fool ourselves (Jeremiah 17:9), but we cannot fool God (Psalm 44:21).

Chapter 4 encouraged Christians to seek God's will through obedience. The most powerful tool we have in this task is the Word of God (Hebrews 4:12). Like a razor-sharp blade that can even separate the joints and ligaments of an animal, the Bible distinguishes between even the most closely-related thoughts of our hearts. This, then, is how we are meant to know the difference between truly godly actions and selfish piety (Matthew 7:21–23). God's judgment will make it clear what is worthy and what is not (1 Corinthians 3:11–15; 2 Corinthians 5:10).

The Bible is an important tool to use. According to this verse, nobody is beyond God's knowledge. We must come to God, through Christ, in faith for salvation (Hebrews 2:9–10). We must be diligent in studying our faith in order to avoid catastrophe (Hebrews 2:1–4). We must be obedient to God in order to fulfill our "rest" and find our spiritual inheritance (Hebrews 3:12–14). And ultimately, we are accountable to God and God alone. This makes the Word of God (Hebrews 4:12) our most important tool as we strive to "hold fast" to our faith (Hebrews 3:6).

At the same time, and critically important to the Christian faith, God sympathizes with our struggles. We can seek mercy and forgiveness, with confidence, specifically because we know Christ has endured the same temptations we have. This is the message of the final three verses of chapter 4.