What was the church lacking before the edict that would allow them to legally counter heresy?
The first known usage of the term in a legal context was in AD 380 by the Edict of Thessalonica of Theodosius I, which made Christianity the state church of the Roman Empire. Prior to the issuance of this edict, the Church had no state-sponsored support for any particular legal mechanism to counter what it perceived as "heresy". By this edict the state's authority and that of the Church became somewhat overlapping. One of the outcomes of this blurring of Church and state was the sharing of state powers of legal enforcement with church authorities. This reinforcement of the Church's authority gave church leaders the power to, in effect, pronounce the death sentence upon those whom the church considered heretical.