Cersei Lannister’s revenge on the High Sparrow and the Church of the Seven cost her dearly, and she only has herself to thank.
In season four when the Church of the Seven make their agenda known by arresting the Septon (in a whorehouse) and make him do the walk of atonement, while everyone expects the crown to intervene to quell the rising fanaticism, Cersei sees an opportunity. She has lost her standing and her power due to Tommen’s marriage to Margery Tyrell. Margery is now the Queen, and she is…the Queen Mother. Without power or influence.
Cersei travels to Flea Bottom to meet the religious leader. Masquerading as devout to win his trust, she says:
She invites the High Sparrow to a meeting at the Red Keep, during which she arms the Faith Militant and denounces Loras Tyrell as a homosexual. By the end of season 6, Cersei has been imprisoned, publicly humiliated and is facing a trial. Cersei’s plan (which isn’t much of a plan) is to demand a trial by combat and unleash the Mountain on whoever fights for the Church. Case closed. As usual, she has miscalculated. Thanks to the Sparrow’s influence on the King, trials by combat are outlawed. To twist the knife, the Sparrow feeds Tommen Cersei’s exact words to recite after he makes the decree, as a sort of middle-finger to Cerssei.
After that, Cersei plots her fiery revenge, but her words come back to bite her.