 During the reign of Queen Mary I of England, Christ Church became a centre of Catholic support, and Harrison converted to Catholicism.Harrison claimed that he returned to Protestant belief before Mary's death in 1558 after hearing the words of Cranmer, Ridley, and Latimer, three Protestant martyrs burned at the stake in Oxford.On 15 February 1559, prior to the award of his bachelor's degree at Oxford, Harrison was instituted as the rector of Radwinter in Essex, by the appointment of Lord Cobham, who owned the right, and to whom he was also household chaplain.