During the period of Byzantine history known as Iconoclasm, Emperors Leo III and Constantine V had banned the worship of religious icons and figures, believing these to be in direct violation of one of the Ten Commandments and in support of idolatry. The Council of Hieria, convened in AD 754 under Emperor Constantine V and initially claimed to be the Seventh Ecumenical Council, had decreed that the veneration of religious icons should be banned, ushering in the Iconoclasm which saw many displays of religious art destroyed. The Iconoclasm also enforced persecution of those who worshiped religious icons as well as monks on a general scale.
When Patriarch Tarasius had succeeded Paul IV in AD 784, he attempted to enter into communion with other churches and believed that religious imagery should be reestablished. In order to overturn one Ecumenical Council, another would need to convene. Pope Adrian I responded to the Patriarch’s request and sent a legate to attend this council, though it would be interrupted by Iconoclastic soldiers in 786. The locale for the council would be moved to Nicaea in AD 787, assembling in the Hagia Sophia.
By the end of the proceedings, the Second Council of Nicaea had overturned the proceedings of the Council of Hieria by declaring that figures such as Jesus Christ, the Virgin Mary, angels and all other manner of religious figures existed as symbols and embodiment not dissimilar to the cross in terms of their importance to Christianity. Thus, they deserved to be revered and venerated in a similar manner as the cross itself, although they were explicit in reserving worship for the Divine Being. The Council also found that the Council of Hieria was not ecumenical in its nature, as representatives from the Western parts of the Empire had not been present to convene, and all of its decrees and rulings were effectively null and void.
The papal legates did not hesitate to support the findings of the Second Council of Nicaea, and a full transcript of its proceedings were sent to Pope Hadrian I. This gathering is regarded as the true Seventh Ecumenical Council and is celebrated in the Eastern Orthodox Church.
Despite the Second Council of Nicaea being regarded as the last of the First Seven Ecumenical Councils, its efforts at reinstating veneration and adoration of holy figures to counteract Byzantine Iconoclasm were short-lived. Under Emperor Leo V in AD 815, Patriarch Theodolos ordered to convene at the Hagia Sophia regarding the reinstatement of the rulings found in the Council of Hieria. This event had brought the beginning of the Second Iconoclasm. The ruling as well as the Iconoclasm itself was maintained through the rule of Leo V, his successor Michael II, and finally through Theophilus. Upon the death of Theophilus, however, the throne passed to his heir, Michael III. Acting as regent on her son’s behalf, Empress Theodora called for and presided over a gathering in AD 843 that was meant to reinstate icon veneration and the rulings decreed in the Second of Council of Nicaea, this time for good.