Existing answers have covered a few of these concepts, but there was plenty of Christian iconography apart from the devil who promoted evil among mankind.
The island itself, particularly Simon's glade, functions as a kind of Garden of Eden that is gradually corrupted by the introduction of evil. Simon's glade turns into a kind of church because of him. Within, Simon attracts the most idyllic and flighty creatures -
He squatted down, parted the leaves [...] gaudy butterflies that danced around each other.
When Simon is stabbed to death by boys with pointed sticks, we are reminded of a certain 'hero' who was beaten by men with sticks before he was killed on a stick. Further, Simon is the character who arrives at the moral truth of the novel, and because he is killed sacrificially as a consequence of having discovered the truth, his life contains strong parallels with that of Jesus Christ. His conversation with the Lord of the Flies similarly parallels the confrontation between Christ and the devil in Christian theology.
However, the parallels between Simon and Christ are incomplete, so the novel is not a pure Christian allegory. Simon is an unsung, solitary, stammering boy where Jesus was an altruistic saviour. For another thing, Simon lacks the supernatural connection to the divine that is the main characteristic of Jesus. Simon is is wise in many ways, with a high intuitive intelligence and exceptional bravery. Yet he is no son of God, and his death does not bring salvation to the island. Rather, his death plunges the island into deeper savagery and moral guilt. For another, Simon dies before he can tell the boys what he has discovered, while Christ managed to spread his moral philosophy before he was crucified.