From A.D. 325-787, seven great ecumenical councils, comprised of bishops from throughout the Empire (and beyond), were called by the Emperor and held by bishops to settle serious matters of doctrine, practice, and discipline. Most at stake throughout these councils was a proper Christology and the dogma of the Trinity.
From the catechism class of July 27, 2014: The Ecumenical Councils, part 1
From the catechism class of August 3, 2014: The Ecumenical Councils, part 2
From the catechism class of August 10, 2014: The Ecumenical Councils, part 3 (The Seventh Council addressing Holy Icons)
St. John of Damascus, Apologia Against Those Who Decry Icons St. John wrote this defense of sacred images ca. AD 730 during the reign of Leo III, who began a campaign against icons.
Readings for further study (just a brief selection):
To be added later.