The kind of sword to which Paul refers was a short sword, designed for close fighting. It was about two inches wide and about two feet long. This is a reminder that the conflict with the devil is usually fought at close combat, with him drawing near to thrust his darts or fire his arrows.
When Paul wrote this letter, the canon of Scripture was not complete. So his readers would have recognised that he was referring initially to the Old Testament scriptures. By extension, we include the New Testament, now that it is completed, in this title.
Referring back to the Old Testament, there is a danger that we imagine it is an inferior form of divine revelation and can be ignored. It is true that, by itself, it is an incomplete form. Now that the canon is complete, it is still an essential section of divine revelation, one indeed which needed the extended Scripture to make the meaning of its content clear.
Nevertheless, the Old Testament was the Bible of Jesus – about it he says in John 5:39: ‘You search the Scriptures because you think that in them you have eternal life; and it is they that bear witness about me.’ It was the Bible of the apostles which they used in evangelism and edification (Romans 15:4: ‘For whatever was written in former days was written for our instruction, that through endurance and through the encouragement of the Scriptures we might have hope’). It was of the Old Testament that Paul said in 2 Timothy 3:16-17: ‘All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be competent, equipped for every good work.’
When we think of spiritual conflict with the devil, there is much in the Old Testament to help us. In the Old Testament there are descriptions of the origins of the devil and his fall, of his subtle temptations to Eve and Adam, of his cruel assault on righteous Job, of his opposition to the prayers of Daniel. Of course, we see his involvement in the background of many of the problems faced by the psalmists. And we can watch and learn from how God's people resisted the devil's attacks.