When you look at the history of the church's entanglement with the death penalty, and then its recovery, gradually you realize the story is like a parable, a penetrating view into something much deeper. It reveals how the church departed from following Jesus and instead turned to other sources for its ethics.
The early Christian church started out opposing the death penalty and citing Jesus in its ethics. According to James Megivern, author of The Death Penalty: An Historical and Theological Survey, Clement of Alexandria, notorious for accommodating the gospel to the culture, was "the first Christian writer to provide theoretical grounds for the justification of capital punishment." Megivern, a professor of religion at the University of North Carolina, Wilmington, says that Clement "appealed to a rather questionable medical analogy [a doctor amputates a diseased organ if it threatens the body] rather than to anything of specifically Christian inspiration."
From Clement and Augustine through the 12th century, none of the passages justifying the death penalty even hints of a reference to the teachings of Jesus. The Bavarian Law, from the end of the 7th century, stands out brightly in clearly opposing the death penalty, citing Jesus in the Lord's Prayer: "for the Lord has said: 'The one who forgives will be forgiven.'"
Persecution of heretics was the main source of entanglement with the death penalty. After Constantine became the first pro-Christian emperor in the year 312, "emperors passed at least 66 decrees against Christian heretics, and another 25 laws 'against paganism in all its forms,'" according to Megivern. "The violence of the age was extraordinary, and Christians were becoming more and more deeply involved in it....Once Christianity had become the state religion, the imperial values articulated in Roman law tended to overwhelm gospel values."