Melville presents his characters in such a way as to bring out the fundamental conflict between good and evil in the world. He accomplishes this by the use of Christian symbolism.
In the presentation of Billy Budd, Melville alludes to both Adam, the innocent first man, and Christ, the innocent "lamb of God," who was sacrificed to redeem man. And in describing Claggart's character, there are a number of allusions to the serpent that tempted Eve in the Garden of Eden and therefore brought evil into the world for the first time. Claggart embodies the evil impulse that is the domain of Satan.
The Biblical references are sometimes obvious and sometimes subtle, but they are pervasive. That Billy is a Christ figure is suggested in chapter 1, when Captain Graveling of the Rights-of-Man explains how his crew stopped their quarreling when Billy enlisted: "Not that he preached to them or said or did anything in particular, but a virtue went out of him, sugaring the sour ones." The allusion is to Luke 6:19, when the multitude tries to touch Jesus, "for there went virtue out of him, and healed them all." The captain also refers to Billy as a "peacekeeper," which suggests Christ's title as the Prince of Peace.
The Christ symbolism gathers force in the closing chapters. Billy's shipmates revere fragments from the wooden beam from which he was hung as if they were fragments from the Cross of Christ. And the ballad, "Billy in the Darbies" that is composed by one of his shipmates, contains the line, "Sure, a messmate will reach me the last parting cup," which could be taken as an allusion to the Last Supper that Christ shared with his disciples.
In chapter 25, as Billy lies in irons on the upper gun deck, there is a potent visual symbol that brings out the theme of good being at the mercy of evil. Melville emphasizes the blackness of the surroundings. The guns and carriages are all painted black, but "in contrast with the funereal hue of these surroundings the prone sailor's exterior apparel, white jumper and white duck trousers, each more or less soiled, dimly glimmered in the obscure light of the bay like a patch of discolored snow in early April lingering at some upland cave's black mouth." The picture presented to the reader is of innocent goodness surrounded by dark evil.