That doesn't answer my point. Luther didn't make that exception, nor does it necessarily follow that you have to have explicit warrant in the NT for everything you do. And (if you believe Scripture is inspired), the death penalty for heresy was certainly not contrary to the Spirit under the Old Covenant.
However, the idea of putting people to death for heresy in the Christian era was invented by secular rulers, not religious ones, oddly enough.
"nor does it necessarily follow that you have to have explicit warrant in the NT for everything you do"
Where not talking about what type of shoes you wear on Sunday - we ar talking about taking an human life in the name of Jesus Christ. This has turned out to be such a black spot on the history of the RCC, how could you possibly see it as a good thing?
"the death penalty for heresy was certainly not contrary to the Spirit under the Old Covenant."
Different covenant, different rules. Israel was an earthly kingdom and engaged in earthly warfare.
"My Kingdom IS NOT OF THIS WORLD. If my kingdom were of this world, then would my servants fight".