It's called "the Armor of God" because its components are not of our own making, but are the gift of God. Some wrongly credit the "shield of faith" to themselves, but this is a recipe for defeat against the flaming arrows of the evil one. Only a supernatural, God-given faith can withstand those assaults. We rob God and indeed even rob ourselves of great resources in the Christian life when we ascribe any of them to ourselves and not to Christ alone. (Eph 6:10-18)
Scripture frequently refers to the the LORD as our shield (Gen. 15:1; Ps. 5:12; Prov. 30:5), so when we take up the shield of faith it means we rest in the Lord ... in Christ Himself.
The Holy Spirit has joined us to Christ and our supernatural faith is like a shield because it continually lays hold of Christ who covers us with His righteousness which makes us immune to the attacks of Satan. Satan paces around the earth hoping to falsely accuse the brethren as being not worthy of the kingdom of God. Of course if we remembered our own record, he would be right, Of ourselves, in armor of our own crafting we have no defense against Satan's fiery darts. And if we ascribe the shield of faith to something we muster up in the flesh we would be unprotected and completely open to his attacks. But the shield of faith is not something we have to muster up ... it is a piece of armor the Lord has given us to use Only Christ our shield protects us.
In this passage, faith is spoken of as the choicest piece of Armor that God gives us because we read, "Above all...taking the shield of faith" The choicest because faith is the grace whereby the sinner receives justification - where he is declared righteous before God.. The evil one will hurl accusations where he finds weakness, but faith remembers what God has done for us ... that we are clothed with Christ and his perfect righteousness and so none of Satan's accusations will ever stand. In the new covenant in Christ's blood God remembers not to treat us as our sins justly deserve. Before God our sin has been vanquished and He fully accepts us in Christ as His sons. Justification was purchased by Christ's obedience, not ours, so let us ever, therefore, take our eyes off of ourselves. He alone has satisfied the demands of the law (Gal 2:16).
"Faith is a steady and certain knowledge of the divine benevolence towards us, which, being founded on the truth of the free promise in Christ, is both revealed to our minds and confirmed to our hearts, by the Holy Spirit." - John Calvin
"Faith is not confined to our knowing that there is a God, but chiefly consists in our understanding what is His disposition towards us. For it is not of so much importance to us to know what He is in Himself, as what He is willing to be to us." - John Calvin