The quotation is taken from C. S. Lewis's Miracles, pages 162-163, and I am not ashamed to confess that the thought of it, the hope of it, brings tears to my eyes:

For I suspect that our conception of Heaven as merely a state of mind is not unconnected with the fact that the specifically Christian virtue of Hope has in our time grown so languid. Where our fathers, peering into the future, saw gleams of gold, we see only the mist, white, featureless, cold and never moving.

The thought at the back of all this negative spirituality is really one forbidden to Christians. They, of all men, must not conceive spiritual joy and worth as things that need to be rescued or tenderly protected from time and place and matter and the senses. Their God is the God of corn and oil and wine. He is the glad Creator. He has become Himself incarnate. The sacraments have been instituted. Certain spiritual gifts are offered us only on condition that we perform certain bodily acts. After that we cannot really be in doubt of His intention. To shrink back from all that can be called Nature into negative spirituality is as if we ran away from horses instead of learning to ride. There is in our present pilgrim condition plenty of room (more room than most of us like) for abstinence and renunciation and mortifying our natural desires. But behind all asceticism the thought should be, “Who will trust us with the true wealth if we cannot be trusted even with the wealth that perishes?” Who will trust me with a spiritual body if I cannot control even an earthly body? These small and perishable bodies we now have were given to us as ponies are given to schoolboys. We must learn to manage: not that we may some day be free of horses altogether but that some day we may ride bare-back, confident and rejoicing, those greater mounts, those winged, shining and world-shaking horses which perhaps even now expect us with impatience, pawing and snorting in the King’s stables. Not that the gallop would be of any value unless it were a gallop with the King; but how else – since He has retained His own charger – should we accompany Him?