Paul's conclusion to this magisterial letter is akin to a commander addressing his troops on the eve of battle. A good commander wants to ensure that, to the best of his ability within the requirements of the mission, no soldier will be lost. And because the enemy threats are diverse, each soldier is armed with a variety of offensive and defensive weapons. In modern combat, the terminology would include personal body armor, gas masks, MOPP suits and the like; but the principle would be the same. Nothing can be neglected or taken for granted. It's real bloody war out there, gentlemen (and, ladies). Take nothing for granted. Don't let your guard down.
Paul's knowledge of combat equipment was more than theoretical. He had traveled extensively throughout the empire, witnessing Roman soldiers at various outposts, and been at the pointed end of a soldier's cutlass. For two years he has been chained to his guards. He has heard the soldiers telling stories to fill the boredom of the night watches. Surprisingly, his real qualms are not with those who hold him prisoner. The real war is not against the Empire (though sometimes the Empire does evil); the real war then, as now, is spiritual. It is against Satan and his minions scattered throughout the visible and invisible universe. A real sword might have some temporary value in a fight with storm troops; the sword of the Word is our real defense against even more deadly threats.
Go in peace to love and serve the Lord. And don't let your shield sag. There's a war on.