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.
What Hebrews 4:12 says
The word of God has life and power. It is sharper than any two-edged sword, and it judges the thoughts of the heart.
Hebrews 4:12 in context
9There remaineth therefore a rest to the people of God. 10For he that is entered into his rest, he also hath ceased from his own works, as God did from his. 11Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief. 12For 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. 13Neither 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. 14Seeing 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. 15For 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.
Hebrews 4, verses 9–15. Read the whole chapter with John Brown’s notes in the Self-Interpreting Bible.
Hebrews 4:12 in the catechisms
The Reformation catechisms and confessions cite Hebrews 4:12 once as a proof text.