Abdu'l-Baha was a prolific author and correspondent, and because he is held to be an infallible interpreter, his writings and recorded talks carry scriptural authority for Baha'is. Among the best known are Some Answered Questions, a series of table talks expounding Baha'i teaching on God, the prophets, the soul and immortality; the Tablets of the Divine Plan, which set out the missionary expansion of the faith; and The Secret of Divine Civilization, an early treatise on the reform and progress of society.
His most consequential act for the faith's future was to provide, in his Will and Testament, for the continuation of authority after him. He appointed his grandson Shoghi Effendi as Guardian of the Cause and interpreter of the teachings, and confirmed the eventual establishment of the elected Universal House of Justice, together the twin institutions on which the modern Baha'i administrative order rests.
When Abdu'l-Baha died in Haifa in 1921, his funeral drew a great and diverse crowd, Muslim, Jewish, Christian and Baha'i, and officials of the new British Mandate, a measure of the regard he had won across communities. In the transition from the founder's revelation to a functioning worldwide institution, his forty-year stewardship is regarded by Baha'is as decisive.