Hebrews 4:12 NKJV - "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."
As a Christian grows in their knowledge of God's Word there comes a tendency to become "too familiar" with some verses of the Bible. By that I'm referring to the practice of many believers to overlook the verses they've known for years. And this verse, Hebrews 4:12 is one of those passages. The sad reality is that too many believers have never really recognized the impact this passage should have on their lives.
God's written Word is ALIVE and it is POWERFUL. Like good medicine in your blood stream, the Word of God has the ability to penetrate and permeate a person's heart and mind. God's Word works "effectively" in Christians who believe it, 1 Thessalonians 2:13. Medicine is meant to bring healing and, at the same time, destroy disease-causing germs. That's a great picture of the affect the Bible has on a person's life! God's Word can bring health to a believer and it can clean out the damaging affects of the world's influence on that person. Each time you read your Bible expect God to use that Word to accomplish this purpose.
God's Word is also Powerful. It can do that which human words alone can't. When witnessing, it's great to share your testimony and show a person what they need to do to be saved. But it's best to let the person read the Bible for themselves. Psalm 119:130 says, "The entrance of Your Words gives light; it gives understanding to the simple."
The matter of discerning is important, but I'll only make a brief comment now. The power of the Word of God shows us whether are lives are lived for God's pleasure or whether we're living for our own selfish desires. You can be sure God will use His Word to make that clear in your life.