Honen (1133-1212) was the founder of the Pure Land Sect of Buddhism. While devotion to the Amida Buddha, Amitabha, and the recitation of the nembutsu were already prevalent in Japan before Honen's day, there was no particular sect instituted with a concentration on the nembutsu as a singularly sufficient means to liberation.
Honen's Pure Land was quite unique because it focused on a simple, pure devotion and involved none of the elaborate schemes and rituals or abstractions common to Japanese Buddhism before it. In that way Honen's Pure Land was very much a common person's Buddhism as compared to the historically Court-centered practice of the more educated and formal class.
Shinran(1173-1262), Honen's pupil and successor was founder of what is known as the True Pure Land, or shinshu sect, took the popularization of Pure Land even further, himself even marrying and proselytizing to the lay people as a married priest, unheard of in that time though now commonplace.
Honen held that simply by recitation of the nembutsu, "namu amida butsu" alone, and no other discipline, one could attain rebirth in the Pure Land of the West, Amida's refuge. Of course, the catch is to say it with an undivided mind and without a single shred of doubt. Shinran tried to make it a little bit easier for his people, focusing on shinjin, or faith in Amida and complete trust in Amida's Other Power rather than one's own power. He said that it was easier for wicked people to get into the Pure Land because they know they're bad and will trust in Amida rather than their own power. A good person is more likely to want to trust themselves. Shinran also believed that if you could recite the nembutsu a single time with wholeness of mind, that was sufficient, where Honen had emphasized repetition.
As Pure Land has developed it increasingly looks similar in practice to the faith/guilt/surrender based system of Christianity. The Pure Land "church" in Japan is often compared to a protestant church in America.
Some say that Buddha WAS Christ. . .