Full Eph 6:16, Gen 15:1, Ps 91 (selected). Greek note: thyreos (G2375) is an NT hapax, it occurs only here, built on thyra, "door": the door-shaped, full-body shield, not the small round buckler. Polybius (2nd c. BC) describes the Roman scutum in almost these same terms, the large rectangular shield, popularly said to have been soaked in water before battle to extinguish flaming arrows, a widely-preached detail without clear ancient attestation.
The testudo formation (Josephus describes Roman testudo tactics against Jerusalem's walls) illustrates collective-defense, shields-of-faith joined are stronger than any single shield. OT echo: Gen 15:1, "I am thy shield" (ASV, YHWH to Abram), and Ps 91:4, "his truth is a shield and a buckler" (ASV). God himself is the shield; faith is the reception of divine-shield-ness.