After Harold had returned from his brilliant defeat of Harald of Norway in the north of England, he learned quickly of the Norman invasion. He'd been suspecting it for some time, but it fell hard on the heels of victory at Stamford Bridge that he would have to defend his country in the south, as well.
Upon his return to southern England, he soon received word from William's forces that he had been excommunicated by the Pope and that the Normans carried papal blessing to invade England. All evidence suggests that this news utterly demoralized King Harold. While he had been a powerful commander against the Norsemen, upon hearing news of the alleged excommunication, he declared, "May the Lord now decide between William and me" (Howarth, p. 164), and before going to battle, "the terrible rumour was starting to spread that the King was excommunicated and the same fate hung over any man who fought for him" (ibid., 165).
Records of how the battle actually went suggest that instead of the dynamic fighting force Harold had inspired just days before, the English mainly stood in one place and were slaughtered. Harold had been transformed by his betrayal by the Pope, and his defeat by William (which from a purely military standpoint was by no means assured) marked the end of the ecclesial distinctiveness of the English church and its subsequent capitulation to Rome under Norman rule. Lanfranc himself, as Archbishop of Canterbury, led the Latinization and Normanization of the English church, while William brutalized the English people.