I've been struggling with issues of faith recently, and in trying to handle it I've been spending more time with God and in reflection/study. One of the books I've been reading is Beth Moore's Believing God. In that book she points out something that I've never realized before. Ephesians 6 talks very famously about the "armor of God". But look closely at the King James Version of verse 16 (we don't see this wording in other translations).
above all, taking the shield of faith with which you will be able to quench all the fiery darts of the wicked one.
"Above all". Most importantly. Before anything else.
Thought I'm not a "King James only" person, and typically read the NIV translation, I think this particular version has it right. As Moore states, the shield is "the armor's armor". If we lift up our shield, then our enemy's weapons never make it to our helmet or breastplate. Let's say that we didn't have a shield. Then even though our other pieces of armor will give us some protection, it can still hurt and bruise. Ask anyone who has done real historical combat and they'll tell you that they'd rather have the blow hit the shield than the other pieces of armor. The rest of the armor is to handle blows that make it past the shield. And even with that protection you can still take some damage. Over time, those injuries can build up and the armor can be damaged. This is especially true of arrows ("firey darts"). Watch any movie involving historical combat and what do the soldiers do when a rain of arrows comes down? They raise their shields! They don't trust their armor to handle it. In fact, English longbows were specifically designed to penetrate plate armor. A shield keeps the arrows away from the body and the rest of the armor. When the shield is down, the soldier is more vulnerable.
This is especially true in spiritual warfare. Our faith is our first line of defense. Yes, righeousness (breastplate) and salvation (helmet) will protect our souls. But if we raise our faith high we can avoid most of the attacks of Satan. Our faith provides us with protection against his "darts" and keeps them from hitting the rest of the armor and potentially hurting or bruising us. Satan knows that if he can get us to lower our shield (faith), we won't stand for long against him. So I believe that the shield of faith is the most important part of God's armor. Without faith, we can't find salvation, we won't be pleasing to God, and our works won't be effective (go read Hebrews 11 to see more on this topic).
Unfortunately, Bibilcal faith is not easy for some people. I'm pointing at myself right now. I have been suffering lately because my shield has been down, and I can testify to the fact that the devil has been beating the rest of my armor pretty severely. I'm working on raising my faith, but it's not always easy and is a work in progress. At least now I know how important it is to do so.