Whenever you're talking religion in Lord of the Flies, you're talking about Simon. And the key chapter for religious imagery is the one in which he climbs the mountain to discover that the beast is in fact just a dead parachutist, shot down in the war raging back in the real world.
The parachutist himself represents a fall from grace (as perhaps, do the boys themselves, falling in a crashed plane onto the island). Golding deliberately moves, at the end of Chapter 5, from the wailing littluns to the parachutist's descent: civilisation on the island has very clearly descended towards savagery. Here's the parachutist, described just after the littluns have cried:
There was a sudden bright explosion and a corkscrew trail across the sky; then darkness again and stars. There was a speck above the island, a figure dropping swiftly beneath a parachute, a figure that hung with dangling limbs. The changing winds of various altitudes took the figure where they would.
This figure terrifies the boys, and Simon, of course, is their saviour - or he would be, if they didn't kill him (sound Christ-like? It's supposed to). Simon's knowledge of the parachutist would save the boys from fear; but they kill him out of fear, thinking he's the beast. And as he dies, he's uttering the truth - which, ironically, could be both a description of the parachutist and a reference to Christ's crucifixion in the Bible:
Simon was crying out something about a dead man on a hill.
Simon, the redeemer, is himself redeemed by nature once his attempted redemption has failed. In a beautiful description, Golding has the dead body of the boy taken back into the bosom of nature - a kind of redemptive calm after the horror of his death:
The water rose further and dressed Simon's coarse hair with brightness. The line of his cheek silvered and the turn of his shoulder became sculptured marble. The strange, attendant creatures, with their fiery eyes and trailing vapours, busied themselves round his head. The body lifted a fraction of an inch from the sand and a bubble of air escaped from the mouth with a wet plop. Then it turned gently in the water.
Notice, of course, the bright creatures around Simon's head. They'd look a bit like... a halo.