Donald S. Whitney, Spiritual Disciplines for the Christian Life, 1991, p. 127.
You don’t have to serve God long to be tempted to think your work is in vain. Thoughts come that your service is a waste of time. Results are hard to find. Regardless of what you think and see, God promises that your work is never in vain (I Corinthians 15:58). That doesn’t mean you’ll ever see all the fruit of your labors you’d hope for, or that you won’t frequently feel nothing has come of all your efforts. But it does mean that even if you can’t see proof, your service to God is never in vain.
Donald S. Whitney, Spiritual Disciplines for the Christian Life, 1991, p. 127.