Down through the years, lots of books and articles have been written on this particular passage of Scripture. Paul John Di Frances, in his book Reclaiming the Ethical High Ground (Reliance Books, 2002), offers the following insight.
During the time of the twelve Caesars, the Roman army would conduct morning inspections. As the inspecting Centurion would come in front of each legionnaire, the soldier would strike with his right fist the armor breastplate that covered his heart. The armor had to be strongest there in order to protect the heart from the sword thrusts and from arrow strikes. As the soldier struck his armor, he would shout "integritas" [in-teg-ri-tas], which in Latin means material wholeness, completeness, and entirety. The inspecting Centurion would listen closely for this affirmation and also for the ring that well kept armor would give off. Satisfied that the armor was sound and that the soldier beneath it was protected, he would then move on to the next man.
At about the same time, the Praetorians or imperial bodyguard were ascending into power and influence; drawn from the best "politically correct" soldiers of the legions, they received the finest equipment and armor. They no longer had to shout "integritas" to signify that their armor was sound. Instead, as they struck their breastplate, they would shout "Hail Caesar," to signify that their heart belonged to the imperial personage—not to their unit—not to an institution—not to a code of ideals. They armored themselves to serve the cause of a single man.
A century passed and the rift between the legion and the imperial bodyguard and its excesses grew larger. To signify the difference between the two organizations, the legionnaire, upon striking his armor would no longer shout "integritas," but instead would shout "integer" [in-te-ger].
Integer means undiminished—complete—perfect. It not only indicated that the armor was sound, it also indicated that the soldier wearing the armor was sound of character. He was complete in his integrity…his heart was in the right place…his standards and morals were high. He was not associated with the immoral conduct that was rapidly becoming the signature of the Praetorian Guards.
The armor of integrity continued to serve the legion well. For over four centuries they held the line against the marauding Goths and Vandals; but by 383 A.D., the social decline that infected the republic and the Praetorian Guard had its effects upon the legion.
As a 4th century Roman general wrote, "When because of negligence and laziness, parade ground drills were abandoned, the customary armor began to feel heavy since the soldiers rarely, if ever, wore it. Therefore, they first asked the emperor to set aside the breastplates and mail and then the helmets. So our soldiers fought the Goths without any protection for the heart and head and were often beaten by archers. Although there were many disasters, which led to the loss of great cities, no one tried to restore the armor to the infantry. They took their armor off and when the armor came off—so too came their integrity." It was only a matter of a few years until the legion rotted from within and was unable to hold the frontiers. The barbarians were at the gates.
How well we would do to recognize the truth of this illustration. As Christians, especially in this day and age, we must never remove or armor. We must never forget who we serve and why.