The common law was triumphantly created by regal power. In its emergence, that law and that power were already locked into a dangerous struggle with a rival system: canon law and the power of the Church, embodied in the person of Thomas Becket, archbishop of Canterbury. Becket asserted that secular courts had no jurisdiction over clergymen because it was the privilege of clergy not to be accused or tried for crime except before an ecclesiastical court. The Church was attempting to exempt a substantial part of the population from the jurisdiction of royal justice. Canon law was setting up as a rival to common law and threatening to undermine the universal sweep of the latter.

During the reigns of the Conqueror and his sons, English bishops had not been unduly protective of clerics, and in some cases had degraded malefactors and handed them over for secular punishment. Italian ecclesiastical jurisprudence refined but essentially supported that stance. In Bologna around 1140 the jurist Gratian published his Decretum, a compilation and codification of canon law that included a provision of the Second Lateran Council designed to protect the clergy from physical attack. Gratian's followers expanded this protection to create a coherent theory of clerical immunity. Clerks were not to be tried in a secular court, but there were exceptions. Ecclesiastical crimes were the exclusive preserve of the Church, but grave secular crimes committed by criminous clergy could lead to punishment by secular judges after the culprit had been deprived of his orders. This was not very different to the prevailing English custom.

During Stephen's reign, however, the power of the undivided Church in a fragmented kingdom was in the ascendancy, and by the time of his death the clergy had become more distinct as a separate order in society.