|Nam Myoho Renge Kyo in Japanese|
Nam or Namu?
Nam is a contraction of namu; I use both in my chanting. Various NB sects will argue over which is correct. I say it doesn't matter as long as you chant one or the other or both. It's equivalent to arguing over I'm vs I am.
Nichiren studied Tendai in the 13th century which is another form of Buddhism. After he left the Tendai monastery, he continued to study on his own. He concluded that Buddhists should follow the Lotus Sutra. Nichiren wrote many letters to the government of his day accusing the other Buddhist paths of corruption. He also wrote letters to his followers and priests to encourage them.
Why NMRK & Nichiren Buddhism
The Lotus Sutra teaches that everyone has a Buddha nature. Chanting helps to unlock it. Of course, you don't have to chant NMRK or even be a Buddhist. Chanting is only one way to meditate.