Simply not accepting the Christian faith, in Medieval or Middle Ages England, was not a punishable offence unless the person had previously been a Christian, in which case they might suffer the punishment for apostacy.
Heresy required more; the public and, to quote William Blackstone in Book IV of his Commentaries on the Laws of England (1756), obstinacy in the disavowal of Christian doctrines.
Brown defines it as:
“... the offence of holding a false opinion repugnant to some point of doctrine clearly revealed in the Scriptures, and either absolutely essential to a man’s salvation or of essential importance in the Christian faith.
“The penalty for the offence ... used to be death or excommunication....”
Heresy came with its unique punishment, summarized in the law books as "burned with fire".
The Christian Church managed to enlist the support of a long succession of medieval kings in using the king's military power to enforce any church declaration of a person's heresy, which resulted in forfeiture of the convict's property to the Church, as well as being burned alive.
The common law even had a Latin name for the legal writ for burning a heretic: a writ de heretico comburendo, as well as a statute by the same name (2 Henry IV Chapter 15).
Some Islam nations - many of which continue to support an overwhelming overlap of religion upon government - recognize heresy as a crime and given the locked hands between religion and state, actively enforce a prohibition of criticism against the Muslim Faith.
In modern democracies, it is customary to hold, as does Canada in its Charter of Rights and Freedoms, that:
"Everyone has the following fundamental freedoms: freedom of conscience and religion."
- Brown, Archibald, A New Law Dictionary and Institute of the Whole Law for the Use of Students, the Legal Profession and the Public (London: Stevens & Sons, 1874), Page 176
- Walsh, C., Jowitt’s Dictionary of English Law (London: Sweet & Maxwell Limited, 1959), page 905