Okay, this is fairly a complex passage. So here's a line-by-line explanation:
"Both Darkly Comic and Tragic deeply, Guy's biography of the 12th Archbishop of Canterbury Thomas Becket Sir, is a Portrait of a saint with Plenty of Shadows."
This line describes the biography of Thomas Becket, the 12th Archbishop of Canterbury, as being both darkly comic and tragic. The biography portrays Becket as a saint with many shadows, suggesting that he was not a perfect or flawless individual.
"Does it Diminish Becket for us to know that this future martyr in a hair shirt (clothing worn by ascetics) also made sure to keep a fine silk robe handy for his return to Canterbury, a stately progress one chronicler compared to Christ's entry into Jerusalem?"
This line asks a rhetorical question about whether learning that Becket, who was known for wearing a hair shirt as a symbol of his asceticism, also had a silk robe for his return to Canterbury would diminish him in the eyes of readers. The line also describes Becket's return to Canterbury as a grand procession, likening it to Christ's entry into Jerusalem.
"That his abstemious diet was partly the result of a lifelong susceptibility to chronic, and debilitating, indigestion?"
This line explains that Becket's austere diet was not entirely a matter of choice or devotion, but was influenced by his chronic and debilitating indigestion.
"That one of his oldest and closest friends would have found his canonization 'utterly absurd'?"
This line suggests that not everyone viewed Becket as a saintly figure, and that even some of his closest friends did not believe he should be canonized.
"Only if we prefer the black-and-white certainties of hagiography to the convincingly human portrayal of a charismatic, contradictory individual who was, as Guy puts it, 'as prickly as he was smooth... a man with the habits of a hedgehog.'"
This line argues that the complexities and contradictions of Becket's personality make him a more interesting and convincing character than a simplistic hagiography (a biography of a saintly figure that idealizes them). The line describes Becket as both prickly and smooth, and compares him to a hedgehog in terms of his personality and habits.