What is attaining Buddhahood? It is to awaken to Ichinen Sanzen and to achieve a great life transcending the limits of time and space. Therefore, Nichiren Shōnin said,

“The only way seemingly that leads to Buddhahood is the ‘3,000 in one thought’ doctrine.
(Kaimoku-Shō, WNS2, p. 108)

and also,

“Ichinen-Sanzen is like father and mother of a Buddha.”
(Shōmitsubō-gosho 3)

In other words, all Buddhas were born from Ichinen-Sanzen. He also said,

“This is not an immediate attainment of Buddhahood in this world (referring to women having to change before reaching Buddhahood), which can only be possible through the ‘3,000 in one thought’ doctrine.”
(Kaimoku-Shō, WNS2, p.90)