At first mention of the beast, Jack negates it's existence, but he also make the claim that if there is a beast, he and his hunters will track it down and kill it. His words are the words of an unbeliever in the beasts existence because he wouldn't bluster about it if he actually believed.
This changes later in the story when Jack leaves the beast a sacrifice after the hunt. His situation and lack of control in the situation bring him to worship the gods of nature. He seems to have given up on the civilization in which he was raised, and in turn joined a primitive civilzation from the past. Silly dreams and beliefs in the "boogey man" have been replaced by worship and sacrifice in answer to desperation.
Initially, the boys are frightened of the beast. I believe that even Jack is frightened, just too proud to admit it. In the end, it's Simon who comes to understand the reality of the beast and that the beast is not what the others believe it to be; it is not a thing. What Simon eventually comes to realize is that the beast only exists inside of those who permit him entrance; the beast grows and thrives in the hearts of those who literally are overcome by the impossibility of their situation and their ensuing savagery. The worse things get; the worse the boys behave-- the worse they behave; the more real the beast becomes.