Your question touches on one of the dangers of allegorical readings of this novel - that is to say that the comparison we can make between Simon and the figure of Christ that is so clearly alluded to in the novel can only be taken so far. We definitely cannot go as far to say that Simon is Christ. Here are some of the reasons why:
1. Although Simon does make two very accurate predictions of the future (e.g. Ralph's safe return to home and by implication, his own death) Simon does not have the permanent connection with God that Jesus has in the Gospels.
2. Although Simon clearly possesses deep wisdom and an ability to discern what is really happening in the island (especially with the Beast), his death does not bring salvation, like Christ's death did. Rather, it only serves to plummet the boys into even greater extremes of degradation and savagery. Also, Simon, being shy and rather tongue-tied, is not able to stammer out the truth he has uncovered before the boys kill him. Jesus, however was killed precisely because of his success in spreading his "mission".
This indicates that when reading Lord of the Flies we need to be wary about pushing the Biblical parallels too far: we can see that Golding meant Simon and the novel as a whole to echo Christian themes without being strictly tied to exact comparisons. The Biblical themes are clearly an important tool to use to decode the themes of Lord of the Flies, but they are not necessarily the only tool to use.