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Why did Thomas Becket’s actions change after he became archbishop of Canterbury?

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King Henry II and St. Thomas Becket — Henry II Henry II (left) disputing with Thomas Becket (center), miniature from a 14th-century manuscript; in the British Library (Cotton MS. Claudius D.ii).

Upon becoming archbishop of Canterbury in 1162, Thomas Becket’s actions shifted because he transitioned from being the king’s loyal chancellor to a dedicated defender of the church.1

While serving as chancellor, Becket had been a close companion to Henry II and actively supported the king’s efforts to consolidate royal power, even when those policies conflicted with ecclesiastical interests. Henry appointed Becket as archbishop specifically expecting him to continue this cooperation and help bring the church under tighter royal control.12

However, once installed as archbishop, Becket underwent a profound change in allegiance. He became a militant champion of ecclesiastical supremacy and the Gregorian Reform movement, which advocated for the independence of the church from secular authority. This ideological shift led him to oppose Henry’s attempts to restrict church privileges, most notably regarding the jurisdiction over "criminous clerks" at the Constitutions of Clarendon. His refusal to yield to royal encroachment ultimately transformed their former friendship into a bitter, long-standing quarrel.12345