When we become one with the Buddha by practicing the Odaimoku, we enter into the world of the Great Mandala. It also means that we are participating in the ceremony in the air (assembly in the sky) where the Buddha preached the Lotus Sūtra. We are involved in the world of Ichinen Sanzen. It also means that the Great Mandala exists inside of our own bodies. Nichiren Shōnin states in Nichinyo Gozen Go-henji (Reply to Nichinyo):

“Do not try to obtain this honzon just anywhere, for it exists only in the flesh of those who have faith in the Lotus Sūtra and who chant ‘Namu Myoho Renge Kyo.”
(NOPPA Nyonin Gosho, p. 138)