“For the word of God is quick and powerful and sharper than any two edged sword” Hebrews 4:12
The best way to understand this verse is to deconstruct it in the following manner.
The word of God is Jesus.
“In the beginning was the word and the word was God and the word was with God.” John 1:1
There was never a time when Jesus did not exist and was not with God
“And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was.” John 17:5
“Father, I will that they also, whom thou hast given me, be with me where I am; that they may behold my glory, which thou hast given me: for thou lovedst me before the foundation of the world.” John 17: 24
The two edged sword refers to how harsh the unfiltered truth is when you hear it from Jesus. It is to say the truth cuts deep.
“piercing even to the dividing asunder of soul and spirit”
This refers to how Jesus, when looking at you, can separate who you are as a spirit, and what you think as a man.
“and the joints and marrow”
This refers to His ability to separate the things you do in the flesh instinctively, like a knee jerk reaction rather than anything you do that is well thought out beforehand.
“and is a discerner of the thoughts and intent of the heart” Proverbs 24:12
This refers to His ability to separate what you have thought about and what thoughts have made it into your heart, which you intend to act on, whether you had the opportunity to act on those thoughts or not, good or bad.
“and his feet are like fine brass”
Referring to Christ judgment.
Brass is a substitutional alloy. It is used for decoration because of its bright gold-like appearance; for applications where low friction is required such as locks, gears, bearings, doorknobs, ammunition casings and valves; and extensively in musical instruments such as horns and bells for its acoustic properties. It is also used in zippers.
Brass is often used in situations where it is important that sparks not be struck, as in fittings and tools around explosive gases, it is used as a base to make things out of, items like rails or poles which were often covered over with gold. Gold and brass were a symbol in Rome for Cezars, Magistrates and Governors. It was the symbol of authority for judges in Rome.