How do Christians use the “sword of the Spirit”? In this sermon on Ephesians 6:17 titled “The Sword of the Spirit,” Dr. Martyn Lloyd-Jones explains that the sword of the Spirit differs from the other pieces in three ways: it does not protect any part of the body, it is a defensive weapon, and it is also an offensive weapon. The devil can terrorize people into hopelessness but the sword is the antidote. It is possible to resist the devil and cause him to flee. The Spirit is not the sword but provides the sword—the biblical Scriptures—and enables the Christian to use it correctly. A general attack upon the truth is a strategy of the enemy to shake God’s people from the truth. This is done in many ways, including philosophy (humanity can encompass truth and find God out of their own ability and intellect) and knowledge (any knowledge that humanity has, but science in particular). The more one understands Scripture, the better they can identify the errors lobbed at them. If one doubts the truth and authority of the Bible, it is like having a broken sword in hand and they are already defeated by the enemy. Knowing Scripture in a detailed manner and being soaked in the whole of it is necessary to effectively reply to someone who calls truth into question.

http://www.mljtrust.org/sermons/the-sword-of-the-spirit/