Why does Nichiren Daishonin place so much importance on propagation? It is because the great path for each practitioner to attain enlightenment in this lifetime lies in actively striving to spread Buddhism with the same vow for kosen-rufu as Nichiren Daishonin himself.
>From The Wisdom for Creating Happiness and Peace, part 2, revised edition, p. 272

Daisaku Ikeda

Chanting Nam-myoho-renge-kyo is fundamental to this, enabling you to reveal your innate Buddhahood just as you are. Not only will chanting give you a wonderful, fundamental self-confidence, it will also adorn and dignify your life with the brilliance of your true and highest potential. Please have absolute confidence in yourself, because you are living the noblest possible life, with a beautiful heart.
>From The Wisdom for Creating Happiness and Peace, part 1, revised edition, pp. 207-08

Daisaku Ikeda

When our faith is so strong that we love the Gohonzon, love daimoku, and love Soka Gakkai activities, the “benefits of responding with joy” will flow forth limitlessly in our lives.
From The Wisdom for Creating Happiness and Peace, part 2, revised edition, p. 249

Daisaku Ikeda