In addition to all this, take up the shield of faith, with which you can extinguish all the flaming darts of the evil one
Ephesians 5:16 (NIV)
When we think of a shield as part of a set of armor, we tend to think about knights and the shield on the arm protecting the bearer from injury in single combat. But that’s not the type of shield Paul had in mind. Roman shields tended to be fairly large, carried by the infantryman and basically useless in single combat. What the Roman army did was line up and “lock” their shields together, each man’s shield protecting not just himself, but his mates on the right and left. As long as they were able to maintain this shield lock, the odds of getting wounded or killed were pretty low, but let just one man panic and break lock and it provided an opening for the enemy to come in. This is one of the reasons the penalties for desertion were so severe, particular if it happened in combat. The deserter not only endangered his life, he endangered the life of all those around him, by leaving an opening for the enemy.
As Christians, our shield is our faith in God. But that shield is largely ineffective by itself. It must be locked in with the shields of other believers to truly protect us. We must share our faith with other Christians, shielding one another from the fiery darts of the evil one. A solitary Christian with great faith can do great things, but a group of Christians with solid faith, who are “locked” together and in agreement, can do even greater things.