Final answer:
Christianity was made illegal in Rome by Emperor Nero, with Christianity's most widespread official persecution under Emperor Diocletian. The persecution was due to the threat the monotheistic Christian beliefs posed to Roman religious practices. The practice of punishing Christians ceased after the Edict of Milan in 313, issued by Emperor Constantine.
Step-by-step explanation:
The individual responsible for making Christianity illegal in ancient Rome was Emperor Nero, who reigned from 54 to 68 CE. Nero's notorious persecution of Christians began after the Great Fire of Rome in 64 CE, which he blamed on Christians to divert suspicion from himself. According to Roman historian Tacitus, this led to severe punishments and executions of Christians. Later, Emperor Decius in the year 250 called for universal sacrifice, and adherence to this decree was enforced; Christians who refused to make sacrifices to the emperor were punished, so denying Christianity could spare someone from death. The hardest persecution of Christians was ordered by Emperor Diocletian, starting in 303, where not participating in Roman religious practices, which Christians avoided due to their monotheistic beliefs, was considered an act of treason. Diocletian's persecution ended with the Edict of Galerius in 311, and religious freedom for Christians was eventually established by the Edict of Milan in 313, issued by Constantine.