Often people have thought that the councils like that of Nicea 325AD were all about exclusion and making a narrower definition of Christianity because they usually condemned some teaching as heretical, and the first council is no exception because of its opposition to Arianism. But it is my suggestion that these first councils were more about opposing people like Marcion of Sinope who tried to cut Christianity down to something smaller, rejecting the majority of the Bible, for example. After all, the councils did bring together the patriarchs of the church from around the known world to see what they could all agree Christianity was about. It certainly seems to me that the creeds they came up with were pretty minimalist.
To be sure this is not what the later councils were about. Starting with the council of Ephesus in 431 AD branches now grouped in what is called Oriental Orthodoxy were cut off from the rest of Christianity starting with the Church of the East also known as the Eastern branch of the Syriac church over the objections of Nestorius to the idea that Mary is the “mother of God.” Next it was the council of Chalcedon 451 AD which cut off the rest of Oriental Orthodoxy over the teaching of Cyril of Alexandria that human and divine natures were united in Jesus. This trend reaches its ultimate end when the patriarch of Rome decided the opinions of the other patriarchs were no longer needed and we have the Great Schism 1054 AD separating Roman Catholicism from Eastern Orthodoxy over the adoption of the filoque clause into the creed. From then on the RC held its councils by itself to decide what to be considered Christian and to call them ecumenical is rather laughable.
It has been mentioned to me that others have some sympathy for Marcion because so much of the OT is downright nasty as well as hard to swallow. But I think there is much of value in the OT and we simply need the caution that Jesus should be our gold standard and lens through which the OT should be read.