It was solidified by the ecumenical council that met at Nicea in 325. IOW, it was a consensus decision.
consensus? ill add clarity.
Constantine did want unification no matter what the outcome may have been. He called the council because he was upset the bishops made it public after the problem grew so large.
The popular vote did win, but it was far from consensus. Only a small number of bishops even showed up, and the Arian controversy was not small by any means. The Council of Nicaea did not end the controversy either.
Constantine is believed to have exiled those who refused to accept the Nicean creed.
Emperors considered themselves responsible to the gods for the spiritual health of their subjects, and after Constantine they had a duty to help the Church define orthodoxy and maintain orthodoxy.
I understand that his use of force is stated as myth. BUT
In his opening address to the Council at Nicea Constantine referred to God using him to put away "the impious hostility of the tyrants," bringing peace to God's people.
Remember he just got done judging the Donatist crisis years earlier.
http://www.churchhistory101.com/century4-p7.php
A statement was read (perhaps written by emperor) welcoming the bishops and rejoicing that the empire had come to peace. Now it was the intention of the emperor that the Church of the Lord be filled with peace. Rufinus records that Constantine had an attendant bring in an armful of scrolls and letters sent to him from all over the empire. It was announced that these communications were letters of accusations and complaints sent by bishops against other bishops.
Constantine then let the bishops know that he had not read any of them and instructed his attendant to burn them on the altar, saying that he wanted all grievances settled during their council.
This is pretty clear he was judge and jury here, and demanded peace.
Constantine did introduce, argued for and
eventually insisted that the term homoousias be used in a creedal formula from the council that would definitively state the universal position of the Church
Though the emperor was filled with great optimism, many bishops were not as thrilled.
A novice in the faith had pushed for a creed that had contained a key non-scriptural term and had not been well thought-out.