According to Nichiren, admonishing slanderers of the Lotus Sutra is the only practice that leads to Buddhahood in Mappo.

"In our present day, the Lotus Sutra is of course supreme, as it was in the past. And yet, because the way of practicing it differs from age to age, in this era, even if one were to retire to the mountain forests and read and recite it, or live in the villages and expound its doctrines, or observe all the various precepts, or even burn one’s arms as an offering, that person would nevertheless fail to attain Buddhahood." -- Letter to the Lay Priest Ichonosawa

SGI members "live in the villages and expound its doctrines" but they fail to attain Buddhahood because the practice of the Lotus Sutra in this era is admonishing the slanderers of the Lotus Sutra. This is the only practice that leads to Buddhahood in Mappo.