Parliment
"When he was finally asked how he wished to answer the charges, the King knew his moment had come. “I would know by what power I am called hither, by what lawful authority?” he calmly replied.
And there it was, the central question. The judges, called “Commissioners,” had expected the question, but they still held their breath when Charles spoke it aloud. They knew that he knew that there was no law permitting these proceedings. ... They responded that they were empowered by Parliament and that the King was answerable to Parliament. It was not true and still isn’t.
For each of the following three days the King refused to acknowledge the legality of the “Court”; he termed it “power without law.” ..."
“By What Lawful Authority?”
https://www.takimag.com/article/by-what-lawful-authority/