In the year AD 250 the roman emperor Decius issued his famous edict against the Christians. All Roman citizens were required to demonstrate their personal loyalty to the Roman emperor by sacrificing to the Roman gods. They were then issued a legal certificate to attest to their act. Such a certificate was called a libellus, and a few of these have survived. Those who refused would be killed. Christians were in a quandary because they could not in good conscience obey the law.
Decius had come to power because the group of soldiers he was in charge of had rebelled against the policies of Philippus and they proclaimed him emperor. After Philippus was killed, in AD 249, the empire merely capitulated to the earlier revolt.
So, once in power, an illegitimate ruler used his imperial might to persecute and punish believers.
