"If faith should fail, then every grace will be put to flight. Job's patience was wounded when his hand too tired to hold up his shield as a covering.
Similarly, no grace is safe if it is out from under the wing of faith. At a time when Peter's zeal surpassed his faith, Christ kept him from falling from all grace by saying, "I have prayed for thee, that thy faith fail not" Luke 22:32. Peter's faith was the reserve that the Savior took care should be kept in order to recover his other graces when the enemy foiled him, and to deliver him, bruised and broken, from that encounter.
Christ could not do many miracles for His own countrymen "because of their unbelief" Matthew 13:58.And neither can Satan harm the Christian seriously when faith is in its place. It is true that the devil skillfully aims to fight faith above all, because it is the grace, which keeps him from conquering the rest of the graces. Although a saint may be humble, patient, and devout, Satan can easily tear a hole in these graces and break in if faith does not completely cover each piece of armor. But God's design is still our best defense; He causes faith to be the grace, which makes Satan turn and run.
Faith alone gains acceptance with God for all the other graces and their works. Even the obedient Christian who works hard all day does not expect to take his accomplishment home at night and find God's acceptance for the sake of his human effort. It is only by faith that he can present it through Christ to God. We "offer up spiritual sacrifices, acceptable to God by Jesus Christ" 1 Peter 2:5, that is, by faith in Christ. Faith can so prevail with God that He will take even the smallest broken pieces of human effort from its hand. But He takes nothing unless the hand of faith brings it to Him.”
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