From the beginning of the Bab's ministry in 1844 and up to the present, the Babi Movement and then the Baha'i Faith have faced the opposition of the Shi'i clergy. The bloodiest and most tragic events in Babi/Baha'i history have often been the result of the enmity held, and sentences issued, by the religious leadership of that branch of Islam. It should therefore be no surprise that the most important works of Baha'u'llah were revealed to the Shi'i clergy, and in the Kitab-i-Aqdas, for example, Baha'u'llah addresses a few religious authorities such as Shaykh Muhammad Hassan Najafi.
However, the Shi'i clergy are not the only group of religious leaders addressed by Baha'u'llah. Shaykhi figures such as Hajj Muhammad Karim Khan Kermani have been referred to in the Book of Certitude (Kitab-i-Iqan), and he was addressed both in the Kitab-i-Aqdas and the Lawh-i-Qina. Baha'u'llah further addressed the religious leadership of the Bayanis as well as Azali figures such as Mirza Hadi Dowlatabadi.
To prepare a comprehensive list of all those religious leaders who have been addressed in general Tablets of Baha'u'llah, or in particular works by Him, requires thorough research. Nonetheless it is worth noting that one of the most extensive works of Baha'u'llah, which by itself is a book, is his Tablet to Muhammad Taqiy-i-Najafi, known as the Epistle to the Son of the Wolf. His father, Shaykh Muhammad Baqer Najafi, as well as Mir Muhammad Hussein (the Friday prayer leader of Isfahan,) Hajj Mulla Ali Kani, Sayyed Sadeq Sangelaki, and Hajj Mulla Hadi Sabzavari, are just some of the other key religious leaders whom Baha'u'llah has addressed.
It should also be noted that the addressees in the writings of the Baha'i Faith are not limited to the Shi'i nor the Muslim clergy more broadly; leaderships of all religions have been addressed whether by Baha'u'llah, Abdul-Baha, or in the works of the Guardian. Some have even been the recipients of specific Tablets, as is the case with Baha'u'llah's Tablet to Pope Pius IX, the Tablet of Aqdas, the Tablet of Hardegg, which were revealed to the Christian clergy. Zoroastrian religious leaders, such as Manikchi Sahib, have been addressed in several Tablets by Baha'u'llah and Abdul-Baha. Many passages in the Surih-ye-Muluk are revealed to the entire religious leadership.
It may also be added that Mohyeddin-e Qazi of Qaneqein, Abdurrahman Karkuri, from the Sufi orders, have been the recipients of the Seven Valleys and the Four Valleys.
Shaykh Hadi Najmabadi, one of the greatest mujtahids of the Qajar period, was the addressee of one of Abdul-Baha's works; he also wrote to figures within the Sunni religious leadership such as Shaykh Muhammad Abdu and Shaykh Muhammad Bakhit, the Grand Mufti of Egypt.
Finally, a work of Baha'u'llah that deserves our special attention and will be studied with particular emphasis is the Tablet of Burhan, revealed by Baha'u'llah to the greatest mujtahids of the Qajar period.