"All this shall be accomplished by the sword of Him that rideth on the white horse, because the Word of God is LIVING, AND POWERFUL, AND SHARPER THAN ANY TWO-EDGED SWORD! “These things saith He which hath the sharp sword with two edges…” (Rev. 2:12). Do you know what it means that it is a two-edged sword? In the first century there were three primary swords that armies used when they went into combat. There was one sword that was a big, heavy, cumbersome thing, you could only use it with two hands, and you usually rested it on your shoulders, for it was that heavy. You went into combat and you would swing at the enemy with that sword with both hands, you could almost club them to death with it, it was so heavy, and it would cut right through the metal of their armor. The problem with that was, in the first place, it was so heavy that it got you tired; secondly, if you took a swipe at somebody and missed, you were very vulnerable to a counter attack."
"There was a second kind of sword, the kind we usually think of, a sword with a point at the front and one side of it had a sharp edge. You would take that into battle and try to whack the enemy with that one side, pull it back, and try to strike them again. Then someone thought of the idea of sharpening both sides — it was as revolutionary in the days of Rome as the atomic bomb was for us. When a legion went into combat with a sword that had a blade on both sides they had an incredible advantage — the soldier could cut from all directions, forward and backward. Now that’s what the writer to the Hebrews had in mind when He wrote about the Word of God being living, and powerful, and sharper than any two-edged sword of man. The Word of God is alive! The Word of God is powerful! The Word of God is sharp! How effective it is! Without the Word of the Lord man is like a blind man in a dark room looking for a black cat that isn’t there. Man’s sword is a sword of death. It kills. But God’s sword is living — it cuts away and kills what needs to be killed in order to make alive. It is a living sword — yea, a life-giving sword! Isn’t it wonderful!"
Much more spiritual interpretation in the link:
"Christ and His army of sons come riding out of the heavens on white horses and they come to judge and make war. In the vision John saw this judgment and warfare activity limited to “the sword of their mouth.” There are no other implements of war seen in the vision! In other words, this spiritual warfare is performed entirely by the spoken word of God through their lips! And it is not extreme to suppose that only the word of the Lord would have the power to carry out the warfare! Any individual who sought personal revenge, or a demonstration of human ability, or the effort to fight the battle by the same tired old methods of the carnal church systems or the militant nations of earth, would be powerless to perform any such supernatural activity of warfare. Anything less than the redeeming power of God’s word would not have the motivation of love, and therefore the originator would not be God, and the goal would not be deliverance, salvation, blessing, and restoration. Thus it can be seen that it will be only through enlightened, matured, and perfected sons of God that the words of God in LOVE-JUDGMENT can be spoken! For in them alone will the nature of the Father be unmixed with the nature of man! Aren’t you glad!"