Hebrews 4:12,13 For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart. 13 Neither is there any creature that is not manifest in his sight: but all things are naked and opened unto the eyes of him with whom we have to do.
Here is a quote from a computer based Bible commentary on this verse:
"The next 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 rhe_ma not logos, the familiar word used by John in the prologue to his Gospel. 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.
Second, unbelief is detected by the living Lord. Here the pronoun shifts from the impersonal to the personal: And there is no creature hidden from His sight. Nothing escapes His notice. He is absolutely omniscient. He is constantly aware of all that is going on in the universe. Of course, the important point in the context is that He knows where there is real faith and where there is only an intellectual assent to facts."
This comment, although true, is too focused on the negative, judgemental aspect of this passage. One of the key things we are meant to realize is that there is no point in maintaining hypocrisy before God and His son. We are known inside out but because we are known, if we will be honest with ourselves before God, we can find true understanding and help. So, the last three verses of this section of scripture are among the most comforting in the Bible:
Hebrews 4:14-16 Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession. 15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin. 16 Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need.
I hope you have found this helpful.