We all know the sorrowful history of the subject. The very early church would have never felt a need to have a council on the subject, just as they would have been no council declaring that Christians should not commit adultery, or theft. However, assuming that the church did degrade itself into this non-Christian behavior when did a council first condemn it?
The earliest record I could find is at the fourth council of Toledo in 634AD:
The fourth Council of Toledo had declared,... that it was unlawful and unchristian-like to force people to believe, seeing it is God alone who hardens and shows mercy to whom he will. (source, P238)
Unfortunately this small sentiment of liberal thinking did no go that deep as it was partly made to stop the forced Baptism of Jews, yet the council still insisted that those Jews who were 'already baptized by force' should be still compelled to the observance of Christianity. Therefore, we can regard the declarations of this council as only partially admitted the sin it had become so overcome with.
An actual translation into English of the councils decree can be found here;
Praecipit sancta synodus, nemini deinceps ad credendum vim inferre; enim vult Deus miseretur, et quem vult indurat;” — “The holy synod that none hereafter shall by force be compelled to the faith; for God hath mercy on whom he will have mercy, and whom he will he hardeneth.” (Works of John Owen, Volume 14, Page 270)