In the days, long gone in most modern democracies, where the state enforced religious beliefs, it was a crime to have once espoused the Christian faith and to thereafter (1) deny or renounce being a Christian, (2) adhere to another "false" religion, or (3) to have no religion at all.
Roman law first began hunting down and punishing apostates.
The earlier punishments were loss of property but eventually, this was increased to the death penalty, particularly during the Inquisition.
But even in that bastion of common law, England, William Blackstone reports, in Part IV of his Commeniaries, that apostates were "burnt to death".
Indeed, Blackstone's justification of the zeal against apostates is justified by him, in his 1756 publication, with ominous words:
"The belief of a future state of rewards and punishment, the entertaining just ideas of the moral attributes of the Supreme Being, and a firm persuasion that He superintends and will finally compensate every action in human life; these are the grand foundations of all judicial oaths, which call God to witness the truth of those facts, which perhaps may be only known to Him and the party attesting. All moral evidence, ... all confidence in human veracity must be weakened by apostacy."