南無妙法蓮華経 is the daimoku, the chant at the center of practice in every Nichiren school. Its seven characters give homage to the Lotus Sutra. 南無, namu, is devotion, a taking of refuge, and the rest is the sutra's title, the Lotus of the Wonderful Law. The schools write it two ways: Nam-myoho-renge-kyo for Soka Gakkai and Namu Myoho Renge Kyo for Nichiren Shu. Both are one chant.
A calligrapher's daimoku is a devotional hanging. The Gohonzon, the inscribed mandala of the schools, is a different object and comes from within the tradition. On a wall the daimoku is a practitioner's text. As a tattoo it declares faith in a living religion. Which of the two you choose it for is yours.
Nichiren made the chant of these seven characters the ordinary practice of his school in 1260, seven years after first declaring his teaching at Kiyosumi in 1253; the formula itself is older, a line of homage in a Chinese Lotus rite of the sixth century, and in Japan it closes a court poet's prayer for his dead parents in 881.
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 南無妙法蓮華経.