Christians must realize that they are engaged in perpetual spiritual warfare. In this sermon on Ephesians 6:16 titled “The Shield of Faith,” Dr. Martyn Lloyd-Jones reminds that Paul exhorts believers to take up the “shield of faith.” He referred to other pieces of the armor of God as things to “have,” but Dr. Lloyd-Jones explains that this action changes to “take.” The shift is because the first three pieces of armor are fixed firmly to the body, but the shield must be taken up. People do not fight some general evil, but a particular spiritual person and others he commands. What are the “fiery darts”? Commonly, they are doubting thoughts, blasphemous words, and phrases that fill the mind and come to mentally discourage, depress, and defeat. Other darts inflame desires and passions, destroying peace, and contentment. Some are fiery trials of persecution, antagonistic people, or adverse circumstances. The “shield of faith” is the ability to quickly apply belief in answer to everything the devil does or attempts to do. Faith is not merely an intellectual belief or theory, but is always practical to apply truth. Faith never points to itself but to its object. The object of genuine faith is God, His character, and His promises. When attacks come, faith is actively dependent upon God and His grace in Christ.

http://www.mljtrust.org/sermons/the-shield-of-faith/