That Śākyamuni Buddha is in us also means we are In Śākyamuni Buddha. In other words, we are living in the Buddha’s life. These phrases show us the meaning that the Buddha and we are united and seen from the Buddha’s perspective, we are also one with him.

Nichiren Shōnin stated in the Kanjin Honzon Shō,

“Śākyamuni Buddha, the Lord-preacher of this Pure Land, has never died in the past, nor will He be born in the future. He exists forever throughout the past, present, and future. All those who receive His guidance are one with this Eternal Buddha. It is because each of our minds is equipped with the ‘3,000 modes of existence’ and the ‘Three Factors,’ namely, all living beings, the land in which they live, and the five elements of living beings.”
(WNS2, page 148)

In summary, according to the doctrine of Hon-In Hon-Ga (true cause and true effect) and Ichinen Sanzen of Ji, a master (the Buddha) and his disciples (beings of nine realms, especially followers of the Lotus Sūtra) are beginningless and endless and one with the Buddha. This is the world of Ichinen Sanzen of Ji.