When we put our faith and trust in God, He protects us. He literally surrounds us with Himself to shield us from the enemy. It is our faith that holds onto the shield, but God is our actual protector.
When we are surrounded by God, we are surrounded by His love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control, favor, wisdom, truth, and righteousness. Everything is filtered through His presence. Our hearts and minds are protected and nourished through Him.
When we sin, when we stop obeying His commandments, we lower our shield and allow the enemy to attack, and he will take every opportunity to do so. But when we obey and love our God with all our heart, soul, and mind, that shield gets polished to a high sheen that blinds the enemy for miles.
I love the fact that God is my shield. You have hedged me behind and before and laid Your hand upon me. Such knowledge is too wonderful for me. It is high, I cannot attain it. (Ps 139:5-6)