A colt, one or two-years old, does not look sick even if its joints grow long, legs become round and thin, and upper legs are long. When it becomes seven or eight years old and is heavier, all kinds of trouble become apparent as the blood vessels become larger and the upper part of the body grows bigger while the lower part remains small. The horse is no longer useful to people, as it is weak in strength and short in longevity. It is like a small boat loaded with huge rocks or a tiny tree with huge fruits.
The same is true with heavenly beings. At the beginning of the Kalpa of Construction, when the world is being created, gods were born with excellent rewards of virtuous acts in previous lives, and men were not evil. Therefore, heavenly beings were shiny in body, pure in spirit, as bright as the sun and moon, and as brave as the lion and elephant. When the Kalpa of Construction was over and the world entered the Kalpa of Continuance, heavenly beings from the previous period grew old and declined like the waning moon, newly born gods were mostly equipped with inferior rewards of actions in their previous lives. As a result, the three calamities and seven disasters occurred all over the world and people everywhere began experiencing sufferings and joys.
Then the Buddha appeared in this world and prepared the panacea of life, that is Buddhism, for the gods and people. Like oil added to a lamp or a cane supporting an elderly person, heavenly beings regained the authority and power they possessed in the Kalpa of Construction.
Kangyō Hachiman-shō, Remonstration with Bodhisattva Hachiman, Writings of Nichiren Shōnin, Doctrine 1, Page 257-258