The Three Ages of Buddhism, Welcome to Mappo.

The Three Ages of Buddhism, also known as the (Three Ages of the Dharma) are three divisions of time following Buddha's passing.  Buddhist temporal cosmology assumes a cyclical pattern of ages, and even when the current Buddha's teachings fall into disregard, a new Buddha will at some point be born to ensure the continuity of Buddhism. This cosmology  appeared early in Buddhist writings references to the decline of the Dharma over time can be found in such Mahayana sutras as the Diamond Sutra and the Lotus Sutra, but also to a lesser degree in some texts in the Pāli Canon such as the Cullavagga of the Vinaya Pitaka. Nanyue Huisi was an early monk who taught about it; he is considered the third Patriarch of the Tiantai.

The Three Ages of Buddhism are three divisions of time following Buddha's passing:

1. The Former Day of the Law, also known as the Age of the Right Dharma the first thousand years (or 500 years) during which the Buddha's disciples are able to uphold the Buddha's teachings;
2. The Middle Day of the Law, also known as the Age of Semblance Dharma, the second thousand years (or 500 years), which only resembles the right Dharma;
3. The Latter Day of the Law , Mappo, which is to last for 10,000 years during which the Dharma declines.

             The three periods are significant primarily to Mahayana adherents, particularly those who hold the Lotus Sutra in high regard, such as the Tiantai and Tendai  and Nichiren Buddhist, and some schools of Zen,  who believe that different Buddhist teachings are valid (i.e., able to lead practitioners to enlightenment) in each period due to the different capacity to accept a teaching  of the people born in each respective period.  

      In the Lotus Sutra, Visistacaritra is entrusted to spread Buddhist law in this age and save mankind and the earth. He and countless other Bodhisattvas, specifically called Bodhisattvas of the Earth (of which he is the leader), vow to be reborn in a latter day to re-create Buddhist law, thus turning the degenerate age into a flourishing paradise. Shakyamuni entrusts them instead of his more commonly known major disciples with this task since the Bodhisattvas of the Earth have had a karmic connection with Shakyamuni since the beginning of time, meaning that they are aware of the Superior Practice which is the essence of Buddhism or the Dharma in its original, pure form some call the era of Maitreya (the future Buddha)

             Pure Land Buddhism in China and Japan believe we are now in this latter age of "degenerate Dharma". Pure Land followers therefore attempt to attain rebirth into the pure land of Amitābha, where they can practice the Dharma more readily. Nichiren Buddhism has taught that its teaching is the most suitable for the recent Mappō period. Vajrayana Buddhism taught that its teaching would be popular when "iron birds are upon the sky" before its decline^. The Kalacakra tantra contains a prophecy of a holy war in which a Buddhist king will win. Theravada Buddhists taught that Buddhism would decline in five thousand years^.  Some monks such as Dōgen and Hsu Yun had alternative views regarding dharma decline. Oddly enough Dōgen believed that there is no mappō while Hsu Yun thought mappō is not inevitable. Maybe that's why Dōgen  wrote huge volumes on Buddhism while telling his students scholars went to the hell of hungry Ghosts.

So the point of this small bit of trivia is being presented so that you understand that Zen Buddhism in Japan and China, pure land Buddhism in Japan and China, Nichiren Buddhism as they all exist today all developed, more or less,  during what is known as the Kamakura Period of Japan  1192-1333, based on the belief that we have now entered the Mappo period of Buddhism. With the basic understanding that in this degenerate age the people born herein  are not capable of understanding Buddhism as it was taught by the Buddha. 

           So! Each school is in effect a chopped down readers digest form of Buddhism aimed at the diminished capacity of the people who are born in this age because they’re not capable of digesting the pure Dharma.

           Therefore, Zen practitioners are taught to just sit, pure land practitioners are taught to just chant the name of the Buddha of pure light, Amitabha, so that he can take them to a pure land where they can do what’s necessary to become enlightened, Nichiren Buddhists only have to recite the first couple of paragraphs of the Lotus Sutra, over and over, they don’t even have to understand the words.

Just thought I’d mention this, it seems like something worth knowing.