In 325 C.E., just 12 years after the Emperor Constantine declared Christianity to be a legal and acceptable religion, he convened the Christian bishops from across the Roman Empire at the Council of Nicaea to come to agreement about the official doctrines of the church. He was particularly concerned about the divisions in the church in relation to a way of thinking called Arianism, which held a view of Jesus as not being co-eternal with the Father and thus being distinct from and subordinate to the Father.
Arianism was a problem for Constantine because it was in the interests of imperial order that the church be of one mind on its views of the divinity of Christ. At the Council of Nicaea, the bishops rejected Arianism in favor of the more dominant view that Jesus the Son was of the same essence and substance as the Father and thus equal in divinity to the Father. Arianism was deemed heresy and condemned by the Council.
There is nothing wrong or inappropriate about church leaders coming together to seek common understanding and agreement about their views concerning the divinity of Jesus. That is not the problem with the Council of Nicaea. The sin of Nicaea is not the seeking of common understanding, rather it was what was done to those who dissented from the majority view.
At the Council of Nicaea, the empire approved Christian church committed the grievous sin of allowing secular authority to enforce adherence to its doctrines. Penalties included exile of the bishops who held on to their Arian views and capital punishment for any persons who were found to possess Arian writings. Constantine required that all extent Arian writings be burned.
At the Council of Nicaea, the empire approved Christian church lost its soul by accepting capital punishment by the state as a punishment for not adhering to its doctrines. What started as a movement by one who was wrongly executed by the Roman Empire now became an institution maintained by the threat and actual use of wrongful executions by the Roman Empire. Church doctrine in effect became a tool for the enforcement of imperial order as opposed to being the Good News of liberation for the poor and oppressed.
Under the guise of imperial protection, Constantine effectively turned the empire approved Christian church into an instrument for maintaining imperial order, with differing theological views being suppressed by exile or execution. What was once the persecuted church became an imperial instrument of persecution to eradicate divisions that Constantine viewed as problematic for imperial peace.
The empire that killed Jesus now killed people to enforce adherence to empire approved church doctrine. A movement of persons who followed the executed one now became an institution that embraced the empire executing people on its behalf. By accepting the violent state enforcement of church doctrine, the empire approved church lived by the sword rather than by the way of love and justice, thus becoming the very antithesis of the Jesus it claimed to follow. This is the sin of Nicaea.
When the bishops at the Council of Nicaea in 325 C.E. allowed the use of exile, execution, and book burning by Emperor Constantine to enforce their favored doctrines, whatever small part of the institutional church that was still holding on to the hand of Jesus let go and firmly grasped the hand of the same empire that murdered Jesus, thus empowering the empire to sickeningly and grotesquely murder innocents in the name of Jesus, something empire has done time and time again throughout the history of Christendom.