Can you explain Hebrews 4:12, especially phrase: "the dividing asunder of soul and spirit, and of the joints and marrow"?
"Therefore let us be diligent to enter that rest, so that no one will fall, through following the same example of disobedience. For the word of God is living and active and sharper than any two-edged sword, and piercing as far as the division of soul and spirit, of both joints and marrow, and able to judge the thoughts and intentions of the heart. And there is no creature hidden from His sight, but all things are open and laid bare to the eyes of Him with whom we have to do" (Hebrews 4:11-13).
One way to understand a phrase, is to first step back and see the context and what is the overarching point that is being made. Hebrews 4:12 starts with "for," meaning that it is an other point being made in an argument. The writer is stating that we need to put strong effort into reaching heaven. We don't want to be among those who did not make it because of their disobedience. Why? Because the law we obey, the word of God, is able to make distinctions and nothing is hidden from God.
The law is not a dead law, it is living and currently active. It applies today to our lives. This counters the argument that the Bible is a 2,000 year old document that was written for a culture that no longer exists and has to be modified to fit modern society.
It can penetrate deeper than any two-edged sword, reaching deep into a man's spiritual heart to lay bare the things hidden there. It is a two-edge sword because it cuts two ways. If you think you are going to wield it as a weapon against others, you need to understand that it will cut you too. "Therefore you have no excuse, everyone of you who passes judgment, for in that which you judge another, you condemn yourself; for you who judge practice the same things. And we know that the judgment of God rightly falls upon those who practice such things. But do you suppose this, O man, when you pass judgment on those who practice such things and do the same yourself, that you will escape the judgment of God?" (Romans 2:1-3).
The word reaches the soul and spirit, where no sword can reach, and it is able make a division between the soul and spirit. We are told that man is composed of a body, soul and spirit (I Thessalonians 5:23). The soul is the life, whether in an animal or a man. When the soul is separated from the body, it dies. If the soul is separated from the spirit, it too would die. "Do not fear those who kill the body but are unable to kill the soul; but rather fear Him who is able to destroy both soul and body in hell" (Matthew 10:28). The "joints and marrow" refer to a person's physical life since life is in the blood (Leviticus 17:11).
With such deep penetration and dire consequences, it should not be surprising that God's word is able to weigh more than our actions, it is able judge our thoughts and intentions. "Therefore do not go on passing judgment before the time, but wait until the Lord comes who will both bring to light the things hidden in the darkness and disclose the motives of men's hearts; and then each man's praise will come to him from God" (I Corinthians 4:5). God doesn't just judge what we do, but considers why we acted as we did. With such close scrutiny of our lives, we can't approach righteousness casually. It will take hard effort to reach heaven. "And if it is with difficulty that the righteous is saved, what will become of the godless man and the sinner?" (I Peter 4:18).