For which cause we faint not; but though our outward man perish, yet the inward man is renewed day by day. 2 Corinthians 4:16
God has allowed the finger of decay to come to your body and lay you completely aside, and you begin to see what a slight hold you have on life, and the thought comes -- "Well, I expect I will have to cave in. I have not the strength I once had; I can never do the things I thought I would for God."
It may come in a hundred and one ways and you realize that the outward man is wasting, that you have not the might you once had, and this is where the cowardly surrender is apt to come in - only we give it another name. The great craze today is to be healthy - "a sound mind in a sound body." Very often the soundest minds have not been in sound bodies, but in very shaky tabernacles, and the word comes "though our outward man perish, yet the inward man is renewed day by day.
Paul faces the possibility of old age, decay, and death with no rebellion and no sadness. Paul never hid from himself the effect which his work had upon him; he knew it was killing him, and, like his Master, he was old before his time; but there was no whining and no retiring from the work. Paul was not a fool; he did not waste his energy ridiculously, neither did he ignore the fact that it was his genuine apostolic work and nothing else that was wearing him out. Every wasting of nerve and brain in work for God brings a corresponding uplifting and strengthening to spiritual muscle and fiber.
If the outward man is perishing because of an injudicious waste of physical strength or because of wrong habits, then it will always make us faint, or cave in; and if we give up prayer and communion with God, then the decay goes on to a terrible extent; there is no corresponding inward weight of glory, no inner winging.
The apostle Paul continually had external depression, he had agonies and distresses, terrible persecution and tumults in his life, but he never had the "blues," simply because he had learned the secret that the measure of the inner glory is the wasting of the outward man. The outer man was being wasted. Paul knew it and felt it but the inner man was being renewed. Every wasting meant a corresponding winging on the inside. Some of us are so amazingly lazy, so comfortably placed in life, that we get no inner winging. The natural life, apart altogether from sin, must be sacrificed to the will and the word of God, otherwise there is no spiritual glory for the individual. With some of us the body is not wearing away. Our souls are stagnant and the vision spiritually is not getting brighter. But once we get into the heavenlies, live there, and work from that standpoint, we find we have the glorious opportunity of spending all our bodily energies in God's service and corresponding weight of moral and spiritual glory remains all the time. No matter how wearied or expended the body may be in God's work, there is the winging of the inner man into a higher grasp of God.
We have to beware of the pagan notion that our spirit develops in spite of our body; it develops with our body, and the way that spiritual insight develops in the worker is, as Paul states here, in the wasting of energy for God, because in this way the inner man is renewed. It is not a question of saying "Oh, my body is so lazy, I must drag it up to do something," but a question of working on God's line to the last lap, spending and being spent for one purpose only, and that purpose God's. If we put the body and the concerns of the body before the eternal weight of glory, we will never have any inner winging at all. We will always be asking God to patch up this old tabernacle and keep it in repair. But when the heart sees what God wants, and knows that the body must be willing to spend and be spent for that cause and that cause alone, then the inner man gets wings.
Oswald Chambers, The Love of God