act of faith

Behavior that shows or tests a person's religious or other convictions, as in Rock climbing with a new, inexperienced partner was a real act of faith. The term is a translation of the Portuguese auto da fé, which referred to the sentencing and execution of heretics (often by burning at the stake) during the Inquisition, when punishing heresy was thought to constitute an assertion of faith. In modern times it is used for more benign circumstances. [Early 1700s]