Short answers, pulled from the story.
Stephen Gardiner was an English Catholic bishop and politician who lived from 1483 to 1555. He served as Lord Chancellor under Queen Mary I and was the most important religious conservative to retain his diocese during the later reign of Henry VIII. He supported Henry's claim as Supreme Head of the Church of England while opposing Protestant doctrinal reforms.
De vera obedientia, published in 1535, was Gardiner's treatise defending royal supremacy over the Church of England. It was considered the most able of all vindications of that doctrine and included the statement that princes ought to be obeyed by commandment of God without question. Gardiner later recanted it during the reign of Mary I.
Gardiner was imprisoned because he refused to accept the radical Protestant reforms introduced by Lord Protector Edward Seymour during Edward VI's minority. He was sent to the Fleet prison after resisting ecclesiastical visitation of his diocese, then transferred to the Tower of London in June 1548. In February 1551 he was formally deprived of his bishopric and remained in the Tower until Mary I's accession.
As Lord Chancellor, Gardiner placed the crown on Mary I's head at her coronation, opened her first parliament, and negotiated her marriage treaty with Philip II of Spain. He presided over the House of Lords when the heresy laws were revived and sat in judgment on Bishop John Hooper and other Protestant preachers who were subsequently burned.
Gardiner's reported last words were the Latin phrase Erravi cum Petro, sed non flevi cum Petro, meaning "I have erred like Peter, but I have not wept like Peter." He died at Westminster on the 12th of November 1555 and was eventually interred at Winchester Cathedral following a final funeral service on the 28th of February 1556.
In Hilary Mantel's Wolf Hall, Bring Up the Bodies, and The Mirror and the Light, Gardiner appears as an implacable opponent of Thomas Cromwell. In the BBC television adaptation of Wolf Hall, Gardiner is played by Mark Gatiss; Alex Jennings takes the role in the second series, based on The Mirror and the Light.