Why Was Landry Not Excommunicated?

Landry, almost excommunicated and burned at the stake, was only spared because his mother shared a secret to the Pope about the Grail. Gawain got one last shot in by smashing Landry's leg with a warhammer right before they learned that the Holy Grail had once again been stolen from right under their noses.

What was the secret about Landry in Knightfall?

He had a secret affair with Queen Joan, under the nose of his friend King Philip IV of France, in which a child was produced. Once King Philip learned of the secret affair, he declared the Templar Order enemies of the state and ordered them to be captured or killed.

What did Landry's mom tell the pope in Knightfall?

Before Landry can be excommunicated though, his mother speaks with the Pope and tells him something about the Grail and its power that immediately makes the Pope change his mind about Landry's fate.