Because England was Christian, they really did not want to see her as sent by God to defeat them, but rather attempted to paint her as a witch and Heretic.
When she came under English arrest (after the French king failed to ransom her), the church trial, or Inquisition, had real political implications. If the church said Joan was a witch, the English were on God's side; if the church said she was faithful and her voices were worthy of belief, then the French were on God's side.
The head of the trial was the local bishop who was committed to the English side. He trounced legal procedures, intimidated those called to rule on it, and repeatedly ignored Joan’s appeal to the pope, which she had a clear right to do. It was a travesty of justice, and Joan was condemned of heresy, burnt at the stake.
Thirty-so years later, the court records were examined and Joan was posthumously vindicated of all charges of heresy. She was canonized in 1920.