other · 385 · Trier
Priscillian, a Spanish bishop with an ascetic following, was accused of Manichaeism and sorcery, and the case ended in front of the usurper Maximus at Trier. Martin came and argued two things: that it was enough to depose a heretic and monstrous to kill him, and that a secular ruler should not be judging an ecclesiastical charge at all. He got a promise, left, and the promise was broken — Priscillian and several companions were beheaded, the first Christians put to death for heresy by a Christian government. Martin came back to Trier under compulsion later and communicated once with the bishops who had pushed for it, in order to obtain a pardon for others, and Sulpicius says he regarded that one act as the failure of his life and never afterwards attended a synod. This dataset carries the executions nowhere else, and it carries the objection nowhere at all until now.
To stop the emperor executing men for heresy, and to stop bishops asking him to.
The executions happened; Martin's objection is on the record; and the principle he argued has been quoted in that argument ever since.