Soka Gakkai is a lay-based, modern Nichiren practice that was founded in Japan in 1930 by Tsunesaburo Makiguchi and Josei Toda. The main belief or philosophy of Soka Gakkai is based on the teachings of the 13th century Japanese Buddhist monk, Nichiren.

At its core, Soka Gakkai believes in the inherent dignity and potential of every individual. This belief is rooted in the concept of "Buddha nature," which states that all beings have the potential to attain enlightenment and become Buddhas. This belief is also reflected in the Soka Gakkai motto, "Human Revolution," which emphasizes the importance of personal growth and transformation.

Soka Gakkai's main belief is also centered around the Lotus Sutra, a key Buddhist scripture that is considered to be the highest teaching of the Buddha. The Lotus Sutra teaches that all people have the potential to become Buddhas, regardless of their social status or background. This belief is reflected in Soka Gakkai's emphasis on inclusivity and diversity, as the organization welcomes people from all walks of life to practice together.

Another important aspect of Soka Gakkai's philosophy is the concept of "oneness of mentor and disciple." This belief is based on the relationship between Nichiren and his disciple, Nikko, and emphasizes the importance of having a mentor to guide and support one's spiritual growth. In Soka Gakkai, this mentor-disciple relationship is embodied by the successive presidents of the organization, who are seen as the embodiment of the Buddha's wisdom and compassion.

Soka Gakkai also places a strong emphasis on the concept of "humanistic Buddhism," which seeks to apply Buddhist principles to daily life and society. This includes promoting peace, cultural exchange, and social justice through various activities and initiatives.

In summary, the main belief and philosophy of Soka Gakkai revolves around the inherent dignity and potential of all individuals, the teachings of the Lotus Sutra, the importance of the mentor-disciple relationship, and the application of Buddhist principles to daily life and society. This belief system has attracted millions of followers around the world, making Soka Gakkai one of the largest and most influential Buddhist organizations in the world.