This is a good book to get a bit of a walkthrough of the ecumenical councils—what the Church was facing, the various ideas influencing her, and the heresies she fought off. A few concepts that stood out to me as I was reading were that the ecumenical councils deal with teaching around the Trinity and the Incarnation. Groups would fall on either side of a debate too hard and end up presenting a whole host of doctrinal problems regarding the Trinity or the Incarnation.
For example, if Jesus was fully man but not God, then He is simply the man of righteousness, which practically means that everyone must work their way to heaven by pulling themselves up by their own bootstraps—something like Palagianism. And if Christ was fully God but not man, then that means God never united Divinity with humanity, effectively leaving humanity still in a state of needing to be saved. Christ must be both fully God and fully man because, by joining Himself to humanity, He made it possible for humanity to be joined to divinity. Practically all incartnational heresy falls into the trap of emphasising too much of one side at the expense of the other, in one form or another.
There are many nuances regarding these heresies, and from the perspective of someone who hasn't studied them, it may seem like theological nitpicking. But just as an example, Arianism believes that Jesus was a created being. So do Jahovah's Witnesses. That is the type of stark difference a subtle nuance in incarnational theology can make. At at one time, a great majority of Christendom was Arian!
The ecuminical councils had some great theologians and philosophers involved in the shaping of the creeds and canons. This book doesn't get much into the canons. It mostly just walks you through the centuries from an Eastern perspective to help give the reader a feel of what disagreements were alive in Christendom, what they meant, the trajectory of those ideas, why the councils were convened, and what they determined about some of the main heretical themes through the first millennium. It is important to get an Eastern view of how these things took place, because a great deal of these heresies were dealt with and councils convened there.
The teachings of these councils have been tried, tested, and found to be true through the fire of persecution and history. I have a great deal of respect for the tradition that has been delivered to us by our forefathers in the faith and the struggle they had to persevere through in order to preserve the Apostolic deposit. Something I found interesting is that a lot of these debates and the theology proposed during the ecumenical councils came from the East. There were some debates happening in Christendom that the West was simply indifferent to and, to a certain extent, couldn't see the importance of, at least at first, because the questions weren't being asked and thoughts being challenged in the Roman see of Christendom at the time. For example, the Nicene Creed is a summary of the Trinitarian theology of the Capedocian Fathers. So, if you hold to the Nicene Creed, it might be a good idea to read their writings. I think that is a significant piece of history to understand. During this part of history, Constantinople was the "new Rome." She was the "center" of Christendom, and because of this, a lot of doctrine was hashed out under her jurisdiction.
Something I've come to realize is that there is no new heresy under the sun. Everything we see in our modern day is either a heresy that was already battled in the early Church, such as docetism, monoenergism, etc., through ecumincal councils, or is the offspring of one of those heresies. It is incredible how thorough these councils actually were and how much foresight some of these men had who defended the faith for ages to come. We truly are standing on their shoulders.
A council would convene due to tumult and make statements of faith from that tumult. Then, tumult would arise because of the council's statements, debates would occur, and what was said would be tested by Christendom at large. Another council would convene because of the disruption that came from the last council, and what had been proclaimed would either be proven true or false based on what happened among the people and what concerns bubbled up again, all while being tested against the Apostolic deposite as handed down by both written and oral tradition. When looking at this period from a bird's-eye view, you can see that it was a period of doctrinal refinement, and it happened very naturally.
Great book with insightful exposure to the councils. I recommend!