Regarding SGI's assertion that "Attaining Buddhahood in this Lifetime is an important writing" (Expanded).

It was an early writing of Nichiren (1258 or 1259) and Nichiren stated and I paraphrase, "Consider my writings before Sado as my provisional writings". This writing reflects Nichiren's training as a Tientai priest with Chuko (Medieval) Tendai Original Enlightenment principles. Nichiren's latter major and authenticated writings, on the other hand stress, "receiving" from the Eternal Buddha and "keeping".

For example, from his most important writing, The Object of Devotion for Observing the Mind, we read:

"Having profound compassion for those unable to comprehend the gem of the doctrine of three thousand realms in a single moment of life, the Buddha wrapped it within the five characters [of Myoho-renge-kyo], with which he then adorned the necks of the ignorant people of the latter age. The four great bodhisattvas will protect anyone who embraces the five characters as faithfully as T’ai-kung Wang and the Duke of Chou supported King Wen, and as devotedly as the Four White-Haired Elders served Emperor Hui." 

Thus we receive. Regarding "keeping", we read from On Practicing the Buddha's Teachings:

"Life flashes by in but a moment. No matter how many terrible enemies you may encounter, banish all fears and never think of backsliding. Even if someone were to cut off our heads with a saw, impale our bodies with lances, or shackle our feet and bore them through with a gimlet, as long as we are alive, we must keep chanting Namu-myoho-renge-kyo, Namu-myoho-renge-kyo. Then, if we chant until the very moment of death, Shakyamuni, Many Treasures, and the Buddhas of the ten directions will come to us instantly, exactly as they promised during the ceremony at Eagle Peak. Taking our hands and bearing us on their shoulders, they will carry us to Eagle Peak. The two sages, the two heavenly kings, and the ten demon daughters will guard us, while all the heavenly gods and benevolent deities will raise a canopy over our heads and unfurl banners on high. They will escort us under their protection to the treasure land of Tranquil Light. How can such joy possibly be described!"