In 1921, the Swiss psychiatrist and entomologist Auguste Henri Forel — a lifelong sceptic of organised religion — wrote to 'Abdu'l-Bahá with his questions about God. The reply, dated 21 September 1921 and written just sixty-seven days before 'Abdu'l-Bahá's passing, is His last major work on metaphysical matters and one of the most comprehensive treatments of theology and philosophy to come from His pen. Shoghi Effendi described it as one of the weightiest Tablets the Master ever wrote. Forel accepted the Bahá'í Faith after reading it, and spent his remaining years introducing it to fellow scientists, peace advocates, and public figures across Europe.

The Kingdom of Existence gathers the complete correspondence alongside four transcribed talks in which Dr 'Alí-Murád Dávúdí, one of the foremost Bahá'í philosophers of the twentieth century, unfolds the Tablet's central themes: the nature of existence, the relationship between mind and spirit, proofs of the existence of God, and the "philosophers of broad vision" whose insight reached beyond the material world.

Compiler Dr Vahid Rafati supplies an extensive prologue on the historical circumstances of the Tablet and Forel's life, along with a substantial glossary explaining nearly a hundred philosophical, scholastic, and mystical terms — from quiddity and the invalidity of infinite regress to rational soul and transcendence — making a demanding text approachable for the general reader.