Constantinople was the homeland of history also it was the center that Christianity was shaped and become an universal religion. We are going to travel first council of Constantinople that was held in 381.
It was presided by Emperor Theodosius I and it was one of the well-known councils of Christianity. Although the holy trinity was declared in Nicea, some churches continued to question of holy trinity. This council declared the holy trinity definitely.
We can see traces of those councils in old Constantinople.