Pope Innocent III was shocked. It wasn’t merely John’s concessions that upset the pontiff. It was the audacity of the nobility who had used force to get what they wanted.
And there was something else: King John had "given" England to the Pope as a
fiefdom in 1213. Although that action had been opposed by the barons and the English Church - especially
Stephen Langton, then
Archbishop of
Canterbury - Innocent III considered King John to be his vassal. The Pope thought he should have been consulted before the "Great Charter" was signed.
He promptly excommunicated every baron who signed the Magna Carta.
He then declared the "Great Charter" null and void.