Let us ponder this point well, and learn wisdom. Faith is the key to success in the Christian warfare: unbelief is the sure road to defeat. Once let our faith languish and decay, and all our graces will languish with it. Courage, patience, long-suffering, and hope, will soon wither and dwindle away: faith is the root on which they all depend. The same Israelites who at one time went through the Red Sea in triumph, at another time shrunk from danger like cowards, when they reached the borders of the promised land. Their God was the same who had brought them out fo the land of Egypt; their leader was that same Moses who had wrought so many wonders before their eyes: but their faith was not the same. They gave way to shameful doubts of God’s love and power. “They could not enter in because of their unbelief” (Heb. 3:19).
JC Ryle, thoughts on Matthew 17:14-21