I have labored in vain, I have spent my strength for nothing and vanity,yet surely my right is with the LORD and my recompense with my God. (Isaiah 49:4)
One of the pitfalls my father mentioned in his letter of “warning” was the life-quenching discouragement that comes from extended periods of apparent fruitlessness. My father traveled from church to church and dealt with thousands of pastors. He told me about how close so many of them were to throwing in the towel, oppressed by the lifelessness of their people and their own sense of hopelessness.
This is a threat not only to pastors, but to all believers. All of us can become oppressed that our work is of no value. Any one of us can be crushed by the feeling that others do not approve of how we do our work. Who has never felt the pang that he has labored in vain and spent his strength for nothing? When discouragement comes in this form, we need a special weapon to fight the fight of faith.
From John Piper