Answer
The Will and Testament of ʻAbdu'l-Bahá is a pivotal document that established the foundation for the administrative structure of the Baháʼí Faith, ensuring its unity and continuity after his passing. It designated Shoghi Effendi, the eldest grandson of Baháʼu'lláh, as the Guardian of the Cause, responsible for safeguarding and interpreting Baháʼí teachings.