"If only Christians who complain about their weak faith would turn their murmuring into an investigation of why it is so weak! It is because faith has missed its meals from the Word. In earlier days you went through many pressures to keep yourself in the fellowship of God's Word; and you were always rewarded for the time taken from other schedules. But now that you have gradually stopped coming to God in His Word, there is a sad change. It is not easy for you to trust Him; and you have little authority over your unbelief.
The best counsel I have is what doctors recommend for healthy bodies. They find out where a patient was born and send him back there. Let me ask you-- if you ever had faith, where was it born and brought up? Was it not in the sweet air of hearing, meditating, and praying over the Word? Go as fast as you can into your native air, where you drew your first Christian breath, where your faith thrived and grew from the beginning.
Look to your conscience. A good conscience is the vessel faith sails in. If the conscience is wrecked, how can faith be safe? Now you know what sins destroy the conscience-- sins deliberately committed or impenitently repeated. Guard against these deliberate sins! Like a stone thrown into a clear storm, they will so muddy the conscience that you cannot see the reflection of the promise.
But even if you have fallen into the pit of sin, do not stay there. The sheep may fall into a ditch, but it is the swine that wallow in it. Therefore, how hard will it be to stir up faith in the promise when your garment is filthy and your countenance smeared with sin? It is dangerous to drink poison, but far more lethal to let it stay in the body for a long time. Although you are a believer, you cannot act on faith until you have cleansed your heart by repentance."
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