Three times [wounded Odysseus] called, as much voice as a man's head could hold,
and three times Menelaos the warlike heard him shouting
and immediately spoke to Aias, who was near by him:
'Son of Telamon, seed of Zeus, Aias, lord of the people...
let us go to [patient Odysseus] through the battle...'
Now Aias came near him, carrying like a wall his shield,
and stood forth beside him, and the Trojans fled one way and another.
Then taking Odysseus by the hand warlike Menelaos
led him from the battle... (XI.462-5, 469, 485-8)
And so the mighty Aias the Greater used his shield to protect Odysseus and pull him back to the safety of the Greek camp.
In classical Greece, warfare was no longer about heroic individual combat, but massed infantry, called a phalanx (seen above). Discipline, holding the line, was key. In this age, soldiers carried large round shields, the upper lip of which rested on their shoulder. Because the shield was on his left arm, a soldier (called a hoplite) was well-protected on the left side, but his right was a little bare. Here he depending upon the man to his right, and in turn the man to his left depended upon him. Thus, if one man ran, the man to his left would be exposed and he too would run. Soon the whole line would fold.
In the Roman Republic, the manipular formation replaced the old Greek phalanx. Shields once again became an individual matter, tall rectangular things that did not overlap much with one's neighbor. But in certain situations (usually when storming enemy fortresses) the Roman legions would sometimes form a so-called tortoise formation (seen below), holding up their shields to create a box, protecting everyone inside.
Well, you can probably see where this is going... St. Paul not only spoke Greek, but could even quote the Greek poets (Acts 17:28). So whether he had in mind the epics of Homer, the battles of classical Greece or the contemporary military practice of the Romans, when he wrote the phrase, "the shield of faith," there would have been a communal quality attached to it. Faith is by its very nature intercessory: it not only protects us from "the flaming arrows of the Evil One," but we are also called to reach out with that faith to protect those around us. It is not always a fun thing to do - indeed, sharing our shield of faith in the midst of battle can be dangerous - but St. Paul seems to be calling us to nothing less.