"Our blessed Savior tells His disciples what wonders they will do if they believe and "doubt not" Matthew 21:21; and that which is "faith without doubting" in Matthew is faith as a "grain of mustard seed" in Luke 17:6. The doubt against which Christ warned His followers is the kind, which tries to steal the assurance of their faith's genuineness.
For example, you may have inward peace but no joy; and this apparent paradox may cause you doubt your faith. The day may be still and calm though not all glorious and bright with sunshine. And although the Comforter may not come with emotional consolations, He has already hushed the storm of your troubled spirit. And true peace, as well as joy, is evidence of "unfeigned faith" 2 timothy 1:5.
Another way doubt tries to cheat the Christian and prod him to deny his faith is through the absence of peace itself. We have peace with God as soon as we believe on Christ but we do not always have peace with ourselves. The pardon may be past the prince's hand and seal, yet not placed in the prisoner's hand. Do you not think the islanders were rash who accused Paul of being a murderer because the snake fastened itself on him? Then why do you condemn yourself as an unbeliever when afflictions and inward agonies fasten themselves on the spirit of the most gracious child God has on earth?
Yet Scripture relates doubt to the strength of faith, not to the existence of it. "O thou of little faith, wherefore didst thou doubt?" Matthew 14:31.These are Christ's words to the sinking Peter, in which he chides doubts and at the same time acknowledges the reality of faith, even though it is very weak. All doubting is evil by its nature; but some doubting, though evil in itself, evidences grace in the person who doubts."
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