Therefore take up the whole armor of God, that you may be able to withstand in the evil day, and having done all, to stand firm. Stand therefore, having fastened on the belt of truth . . . Ephesians 6:13-14, ESV.
In verses 10-12 we observed that our battle is waged in the spiritual realm and our enemy in this “evil day” is the devil, “the prince of the power of the air” (2:2). To stand firm against him, we need the strength of God. How important is it that we fight in God’s strength? Tom Schreiner reminds us:
Jesus Christ now reigns over all spiritual forces (Eph 1:20–23), and believers have been raised from the dead and now sit with Christ in the heavenlies (Eph 2:1–6). Paul, then, does not summon believers to secure the victory. The battle has already been won through the death and resurrection of Christ. Paul is no dualist, seeing the devil and Christ locked in a mortal and equal combat. The devil and his forces are already defeated in Christ. Believers are to put on God’s armor and clothe themselves with the Lord’s strength.
This strength is described by Paul as the armor of God. The first piece of this armor is “the belt of truth.” The Roman soldier, someone Paul was all too familiar with, used his belt to achieve several things. It was wide, protecting his core. It bound his tunic to keep it out of his way in battle. It held his sword. Likewise, the truth is central to the Christian’s strength for battle against his wicked foe. The Christian soldier finds the core of his strength in the truth of God’s Word.
That Paul would start here is no surprise. The enemy is, after all, “the father of lies” (John 8:44). In earlier chapters, commands the church to speak the truth in love, live in integrity with one another, and live in the light, bearing the fruit of truth and righteousness. Truth, an accurate view of reality shaped by God’s Word, is not negotiable in the Christian life.
Sadly, our culture has denigrated the meaning of the word ‘truth’. It has come to mean little more than “my opinion” and “my experience.” It is bolstered by the opinions of alleged experts: journalists, politicians, activists, bloggers, academics, celebrities, and/or scientists. As Christians, we cannot abide this view of truth. We must recognize that “Every word of God proves true; he is a shield to those who take refuge in him” (Proverbs 30:5).
Fasten up the belt of truth. As you do, I pray that you find it a faithful friend in your battle.
 Thomas R. Schreiner, Paul, Apostle of God’s Glory in Christ: A Pauline Theology (Westmont: IVP Academic, 2006), 303.