Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place, and with your feet fitted with the readiness that comes from the gospel of peace. In addition to all this, take up the shield of faith, with which you can extinguish all the flaming arrows of the evil one. Take the helmet of salvation and the sword of the Spirit, which is the word of God.Ephesians 6:14-17 NIV
The fourth piece of armor that Paul refers to is the shield. There were various types of shields the Romans used over the years. What Paul likely would have seen was a large rectangular shield. This shield could overlap with other shields in a tight formation to provide a solid wall. This would provide better coverage for the Roman troops, especially when facing archery.
For the believer, this shield is faith. Faith in the one who has called us and saved us. And the arrows of the evil one are many and varied. Temptations, doubts, fear, insults, loss. Anything that the enemy can use to lessen your effectiveness as a servant of Christ, he will do.
Satan will use people to fire his arrows. The people of his kingdom will be shooting at us. But sometimes, even other believers will fire arrows at us. The circumstances we encounter can also be a source of arrows. Loss of a job. A car accident. Storm damage to your house. Satan likely did not cause those circumstances. But he will use them to his advantage if possible.
But whatever the arrow is that he fires your way, meet it with the shield of faith. Let nothing move you. Stand your ground and trust in the Lord.