THE THEORY of predestination lies at the heart of the Reformation. It seems today a gloomy doctrine and unlikely, on first consideration, to have become the rallying point for the great rebellion that destroyed 1,500 years of Christian unity in a generation. As C.S. Lewis argued, however, late medieval Catholicism had become so obsessed with man's depravity that predestination seemed to many Christians a revelation of divine mercy. God saves men even though they lack the power to save themselves.

This is the somber backdrop against which Richard Marius draws his portrait of Thomas More. It is a large portrait with all the familiar figures and moments of drama: Henry VIII, Wolsey, Cromwell, Erasmus, Luther, Dame Alice and son Roper; the free- wheeling days of Erasmus' Praise of Folly and More's Utopia, and the dark time that More spent in the Tower awaiting execution.

It is also a revisionist portrait. Marius complains that all previous biographies of More have idealized him. Marius wants to get behind the plaster saint to the flesh-and- blood man. His More is a tormented ascetic who married only because he felt incapable of the celibacy required of monks, a melancholic whose wit was a desperate defense against despair, a man obsessed with power but pretending to humility, and an advocate (in Utopia) of toleration, who gloated over the burning of Protestants: "That he did not succeed in becoming England's Torquemada was a consequence of the king's quarrel with the pope and not a result of any quality of mercy that stirred through More's own bosom." Is it possible, Marius asks, that underneath their bluster, both More and Luther were terrified by the possibility of non-meaning: "In the cool detachment of our own religious nonchalance, we may wonder if each might have been drawn by the horrifying suspicion that Christianity might be a myth."

Marius has been associated with the monumental Yale edition of the Works of More since its inception. He has read all of More's vast and mostly forgotten output, and this is both the strength and weakness of his biography.

On the one hand, when he wants to make a point about the ugly violence of More's controversial style he can quote exactly the right scatological passage, so vivid and unsaintly. On the other, Marius' heavy emphasis on paraphrase and analysis of More's books is literary rather than historical. It makes the biography curiously bloodless. One derives a better sense of More the man, rooted in family, profession, and religion -- a human being caught in the tangle of history -- from R. W. Chambers' biography published in 1935, even though Chambers is guilty, as Marius charges, of idealizing his subject.

THE ANALYSIS, it should be added, is generally excellent, but the one-to-one equation of Hythloday's Utopia with More's own beliefs is a conspicuous lapse. Hythloday is not an exemplary hero but a man corrupted by idealism -- almost the opposite of More himself. The description of More by Hall in his Chronicle applies here: "I cannot tell whether to call him a foolish wise man or a wise foolish man, for undoubtedly he beside his learning had a great wit, but it was so mingled with taunting and mocking that it seemed to them that best knew him, that he thought nothing to be well spoken except he had administered some mock in the communication." For Utopia the rule is caveat lector -- reader beware -- and Marius is not a wary reader.

In the end, and almost in spite of Marius' effort to identify his defects, More the saint eclipses More the man. "He died," writes Marius, "magnificently." He fought to avoid martyrdom with every resource which a lifetime in the law had given him, but he knew the cause was hopeless.

During his imprisonment, he wrote a Treatise on the Passion in which Christ comforts him: "Pluck up thy courage, faint heart; what though thou be fearful, sorry, and weary, and standest in great dread of most painful torments, be of good comfort; for I myself have vanquished the whole world, and yet felt I . . . fear, sorrow, weariness, and inward anguish too, when I considered my most bitter, painful Passion to press so fast upon me . . . Take hold on the hem of my garment therefore; from thence shalt thou perceive much strength and relief to proceed."

Perhaps Marius' account of the final months is all the more luminous because the man at its center has been shown to be fallible. At any rate, however God may have disposed of the soul of His good servant, Thomas More's memory remains a light for all those who have suffered for their beliefs -- and continue to suffer -- in the violent darkness of the 20th-century's political prisons.