A more thorough explanation:

An advocate of the faith is a legal term used in the context of a heresy trial. In Ecclesiastical law, an advocate of the faith is a counsel who represents the prosecution in a heresy trial.

During the Inquisition, the advocate of the faith was responsible for presenting evidence against the accused heretic and arguing for their conviction. They were also tasked with defending the Catholic Church's teachings and doctrines.

For instance, in the trial of Giordano Bruno, an Italian philosopher and astronomer who was accused of heresy, the advocate of the faith argued that Bruno's beliefs contradicted the teachings of the Church and posed a threat to the faith.

Another example is the trial of Joan of Arc, where the advocate of the faith argued that Joan's claims of divine inspiration were false and that she was a heretic who should be punished.

These examples illustrate how the advocate of the faith played a crucial role in heresy trials, representing the Church's interests and defending its doctrines against perceived threats.