Nichiren Daishonin states in “On Attaining Buddhahood”:
Even a tarnished mirror will shine like a jewel if it is polished. A mind which is presently clouded by illusions originating from the innate darkness of life is like a tarnished mirror, but once it is polished it will become clear, reflecting the enlightenment of immutable truth. Arouse deep faith and polish your mirror day and night. How should you polish it? Only by chanting Nam-Myoho-Renge-Kyo.