The Shield of Faith:
“Faith is the foundation of Christian character and the security of the soul” It is through Faith that you become a Christian (Faith in Christ). It is by faith that you are saved…not by works. It is by faith (not by sight that you walk the Christian walk and remain in Christ by faith. It is by faith that we are able to accomplish the main purpose that God created us –to bring glory to him, to please him. Without faith it is impossible to please God. If the devil can take your faith from you, everything crumbles, you lose your soul.
Our faith stands on the Word of God. The Word of God is important because that is the only way we develop, build, and maintain that faith. Faith comes by hearing the word of God. That is why in our arsenal of spiritual weapons, we have the shield of faith (Ephesians 6:10-17). That is also why Jesus says to Peter in Luke 22:31-32,
“Simon, Simon (Peter), listen! Satan has asked excessively that [all of] you be given up to him [out of the power and keeping of God], that he might sift [all of] you like grain, But I have prayed especially for you [Peter], that your [own] faith may not fail”
Jesus revealed a very important truth in this verse; to protect Peter from Satan’s onslaught, Peter’s faith is what Jesus needed to focus on. Why? Because faith is the shield of our spiritual warfare. Jesus knew that Satan would target Peter’s faith (the most important defense, the shield), and when that faith is weakened, then the foundation of Peter’s spiritual life will be destroyed, and he will crumble. So Jesus prayed and strengthened Peter’s faith. So we need to do the same: 1) Pray that our faith will not fail and 2) Meditate on the word of God because faith comes by hearing the word of God. Our spiritual life starts with faith in God, everything else is build on faith. Faith is the foundation, the shield. In 2 Pet 1:5, Peter begs us to add other Christian virtues to our foundation of faith. The way the devil destroys unbelievers is that he keeps them from having faith, when a word of God is sown into their lives, the devil either comes and picks it up or he chokes it up so that that word cannot grow and produce the fruit of faith in God which would then save the believer. The good news is that, the devil cannot take away your faith so that you lose salvation if you are truly born again, but he can mess your faith up so that your life is miserable because his fiery arrows will come piercing your heart without any defense.
 E. M Bounds. The Necessity of Prayer, pg 4.