The trial for heresy was politically motivated. The tribunal was composed entirely of pro-English and Burgundian clerics, and overseen by English commanders including the Duke of Bedford and the Earl of Warwick. In the words of the British medievalist Beverly Boyd, the trial was meant by the English Crown to be "a ploy to get rid of a bizarre prisoner of war with maximum embarrassment to their enemies". Legal proceedings commenced on 9 January 1431 at Rouen, the seat of the English occupation government. The procedure was suspect on a number of points, which would later provoke criticism of the tribunal by the chief inquisitor who investigated the trial after the war.
Under ecclesiastical law, Bishop Cauchon lacked jurisdiction over the case. Cauchon owed his appointment to his partisan support of the English Crown, which financed the trial. The low standard of evidence used in the trial also violated inquisitorial rules.Clerical notary Nicolas Bailly, who was commissioned to collect testimony against Joan, could find no adverse evidence.Without such evidence the court lacked grounds to initiate a trial. Opening a trial anyway, the court also violated ecclesiastical law by denying Joan the right to a legal adviser. In addition, stacking the tribunal entirely with pro-English clergy violated the medieval Church's requirement that heresy trials be judged by an impartial or balanced group of clerics. Upon the opening of the first public examination, Joan complained that those present were all partisans against her and asked for "ecclesiastics of the French side" to be invited in order to provide balance. This request was denied.
Joan of Arc interrogated in her prison cell by the Cardinal of Winchester, by Hippolyte Delaroche, 1824, Musée des Beaux-Arts, Rouen, France
The Vice-Inquisitor of Northern France (Jean Lemaitre) objected to the trial at its outset, and several eyewitnesses later said he was forced to cooperate after the English threatened his life. Some of the other clergy at the trial were also threatened when they refused to cooperate, including a Dominican friar named Isambart de la Pierre. These threats, and the domination of the trial by a secular government, were violations of the Church's rules and undermined the right of the Church to conduct heresy trials without secular interference.
The trial record contains statements from Joan that the eyewitnesses later said astonished the court, since she was an illiterate peasant and yet was able to evade the theological pitfalls the tribunal had set up to entrap her. The transcript's most famous exchange is an exercise in subtlety: "Asked if she knew she was in God's grace, she answered, 'If I am not, may God put me there; and if I am, may God so keep me. I should be the saddest creature in the world if I knew I were not in His grace."' The question is a scholarly trap. Church doctrine held that no one could be certain of being in God's grace. If she had answered yes, then she would have been charged with heresy. If she had answered no, then she would have confessed her own guilt. The court notary Boisguillaume later testified that at the moment the court heard her reply, "Those who were interrogating her were stupefied."
Several members of the tribunal later testified that important portions of the transcript were falsified by being altered in her disfavor. Under Inquisitorial guidelines, Joan should have been confined in an ecclesiastical prison under the supervision of female guards (i.e., nuns). Instead, the English kept her in a secular prison guarded by their own soldiers. Bishop Cauchon denied Joan's appeals to the Council of Basel and the Pope, which should have stopped his proceeding.
The twelve articles of accusation which summarized the court's findings contradicted the court record, which had already been doctored by the judges. Under threat of immediate execution, the illiterate defendant signed an abjuration document that she did not understand. The court substituted a different abjuration in the official record.