Decius was Roman Emperor from 249 to 251, when he was killed in battle against the Goths. At his command, the most severe persecution of the church swept over the whole Roman Empire. This ended a period of freedom from persecution for the Christians, during which the church had grown rapidly, with many people had turned to Christ from older Roman religions. The persecution under Decius was the most violent and most widespread persecution the church had yet faced.
It is likely that Decius thought that the drift away from the Roman gods that had taken place under the previous emperors was the cause of the decay and collapse that was currently taking place in Roman society. He believed that to return to the old gods would make Rome the great empire it had been in the past. To make this happen, Decius issued an imperial edict in January 250 commanding that all citizens of the empire had to sacrifice to the traditional Roman gods, for the safety of the Empire and the Emperor. Those who were obedient and performed the sacrifices were issued with certificates (libelli), to show that they had obeyed the order.
For most non-Christians, this would cause no problem of conscience, as the worship of many gods was the normal practice and pattern of thinking. But for Christians, it caused great problems of conscience. If they sacrificed to other gods, it would be seen as apostasy, for which the popular belief was that no forgiveness was possible. However, some Christians were not willing to pay the ultimate price for their faith, and performed the sacrifices, leaving the church and forsaking their faith. Others were able to purchase libelli by bribing corrupt officials without performing the sacrifices, or otherwise to obtained forged libelli. However, many refused to obey the edict and were will to suffer the consequences.
In the persecution, many Christians were killed, some were imprisoned, and others fled. The two main leaders of the church, Cornelius bishop of Rome, and Hippolytus bishop of Jerusalem, were imprisoned and died in prison. Origen was imprisoned, and died in 254 as a result of the injuries inflicted during his imprisonment. Cyprian, bishop of Carthage, fled to safety and spent the period in hiding, for which he was scorned by many.
One problem that arose in the church following the persecution under Decius, was what should be done with people who had denied their faith, but now wanted to return to fellowship. Some were lenient and forgiving, allowing people back following a period of penance. Others took a hard line, refusing to allow lapsed Christians back into fellowship. Some split off from the church in Rome over this issue under the leadership of Novatian, who had been defeated in election as bishop of Rome in 251. He took a very rigid position and criticised the Roman church for being too lax.
On the other end of the spectrum, there were those who were eager for martyrdom, as Ignatius had been. The belief developed that great merit before God was earned by being faithful under torture, and the ultimate spiritual prestige was to be martyred. This led to a cult of worship at the shrines of the martyrs and an adoration of their relics. More seriously, it also led to the issuing of indulgences by those suffering for their faith. The indulgences passed on some of the extra merit earned by their suffering to others who had committed apostasy during the persecution. It was this same question of indulgences that Luther protested against nearly 1300 years later, leading to the Reformation.