Thomas Becket's personality and character have proved notoriously resistant to interpretation, despite the vast amount of testimony to his life and death. There are two particular questions about Thomas which no amount of historical evidence has been able to resolve. One concerns his murder: did Thomas, as his biographers suggest, foresee his death and willingly embrace it? The other, the subject of this chapter, is: how do we explain the change from Thomas the worldly chancellor and friend of the king to Thomas the archbishop, champion of the Church? While this question has exercised many scholars, little attention has been paid to what the biographers say about it. This seems especially remiss in that the interpretation of Thomas's behaviour most frequently discussed – conversion – originates with the posthumous biographers. Nowhere in contemporary letters is it advanced, not even in John of Salisbury's assessment of Thomas's sanctity of early 1171, Ex insperato. The notion of conversion is central to most of the Lives, not only in relation to 1162, but running throughout their portrayal of Thomas from birth until death. Therefore it is all the more important to ask: What do the biographers mean when they say that Thomas underwent a conversion?

Becket's biographers agree that something extraordinary happened when he became archbishop of Canterbury in 1162. Transformed, they write, into a new man, he immediately embarked upon a more religious life and began with new zeal to champion the cause of the Church.