Roman soldiers carried a couple of shields, often. This particular shield probably refers to the large, almost body sized, shield they would carry to protect themselves when an enemy would launch flaming arrows into the air as part of their attack. A soldier could almost literally hide under or behind this shield and be safe.
What are the fiery darts of the wicked? Well, those represent the temptations Satan fires at believers daily. What is the faith about which Paul speaks? It’s not “The Faith,” as in true Christian faith as a system of belief. Here, Paul refers to our own personal faith in God and His strength and ability to protect us.
Our personal faith in God’s word and his promises are what protect us from the temptations Satan launches at us. We have to understand that the promise of pleasure temporarily that Satan offers is far exceeded by the promise of life and joy eternally which God offers.
I present posts about God's Word, with a personal focus on how God's grace is received through saving faith in the Lord Jesus Christ... Sinners must understand and obey the Gospel of our Lord Jesus Christ... See this link for more...