The apostle compares faith to a shield because of a double resemblance between this grace and that particular piece of armor.
The first likeness is that shield is not for the defense for any one part of the body, as most other pieces are. The helmet is fitted for the head and plate designed for the breast, but the shield is intended for the defense of the whole body. Therefore it was to be made very large and was called a "gate" or "door" because it was so long and large that it covered the whole body. And if the shield was not large enough to cover every part at once, this skillful soldier could turn it this way or that way, to stop the swords or the arrows, no matter where they were directed. This resemblance reminds us of the importance of faith in the life of a Christian. It defends the whole man-- every part of the Christian is preserved by it.
Sometimes the temptation is leveled at the head-- at the saint's reasoning. Satan will dispute truth and, if he can, will make a Christian question the validity of faith merely because his understanding cannot comprehend it. And sometimes he prevails, blotting out a person's beliefs in the deity of Christ and in other great and profound truths of the Gospel. But faith intervenes between the believer and this arrow, coming to the relief of the Christian's weak understanding.
Abraham, "being not weak in faith...considered not his own body now dead" Romans 4:19. If reason had had the upper hand in that business, if that holy man had put the promise to a test of sense and reason, he would have been in danger of questioning the truth of it, although God Himself was the messenger. But faith brought him through the test. "I will trust the Word of God," says the believer, "not my own blind reason."
Quoted material from, ”The Christian in Complete Armour Daily Readings in Spiritual Warfare” by Gurnall and James S Bell. http://www.moodypublishers.com/pub_productDetail.aspx?id=41823&pid=53617