The Shield

in addition to all, taking up the shield of faith with which you will be able to extinguish all the flaming arrows of the evil one.
Why is faith a shield? To understand this we must consider Hebrews 11:1 “Now faith is the assurance of things hoped for, the conviction of things not seen.” The word for faith is πίστις (4102) and means to win over persuade. Faith. Subjectively meaning firm persuasion, conviction, belief in the truth, veracity, reality or faithfulness (though rare). Objectively meaning that which is believed, doctrine, the received articles of faith.[1] Faith is a weighting of the evidence, both for and against and reaching a conclusion based on that evidence. So we see that faith is an orientation to reality. But what is reality? Reality is what is actually real, not what we think, or perceive as real. Let me give an example: for thousands of years people thought that the earth was flat, that was perceived reality. What was the biblical view of reality at that time? For that we turn to Isaiah 40:22 “It is He who sits above the circle of the earth, And its inhabitants are like grasshoppers, Who stretches out the heavens like a curtain And spreads them out like a tent to dwell in.” (Emphasis added) So much for perceived reality. It took science a thousand years or so to catch up with the bible.
The Greek word for assurance is ὑπόστασις (5287) and it means to place or set under. In general, that which underlies the apparent, hence, reality, essence, substance; that which is the basis of something, hence, assurance guarantee, confidence.[2] This assurance comes from faith, not wishful thinking, but faith based on proven reality.
Consider Proverbs 30:5 “Every word of God is tested; He is a shield to those who take refuge in Him. “ Faith is a shield because of who our faith is in. Faith in the wrong thing, no matter how sincere is useless. Let me illustrate. I once knew a man whose sister loved fresh picked mushrooms. She knew which ones could be eaten and which could not. One day she ate what she was convinced was a save mushroom to eat. It nearly killed her. Afterwards, if you ask her she would still tell you that she believed it was a safe mushroom to eat. Despite that belief she nearly died. Her faith was useless.
As our trust in God grows we are able to turn aside the enemy’s attacks. Faith is like a muscle though; if you do not use it then it withers. You cannot ignore God every day then expect to be able to trust Him to deliver you in your time of need.

[1] The Complete Word Study Dictionary New Testament
[2] The Complete Word Study Dictionary New Testament