Shoghi Effendi
After Bahá'u'lláh died, Shoghi Effendi's grandfather, `Abdu'l-Bahá lead the Bahá'ís.  `Abdu'l-Bahá died, he left a "will and testament" that said that Shoghi Effendi was the "Guardian", and that his job was to guard and lead the Bahá'í Faith.  `Abdu'l-Bahá's will said that only Shoghi Effendi could interpret `Abdu'l-Bahá's or Bahá'u'lláh's books and letters.  This was to prevent different people from splitting the religion up because they thought Bahá'u'lláh or `Abdu'l-Bahá meant different things, the way many other religions have split up.