I have read the book recently and I don't think Jack and Ralph believe in the beast. They don't think its actually real, but the idea of the beast existing scares them. Jack even says(to cover his fear) that he and his hunters would find the beast and kill it in Ch.5. So, technically, they are scared but they don't show it as much as the littluns because they are older.
In "The Lord of The Flies", Jack tells the boys that the beast came once in disguise and will use this same trick to fool them again. Jack knows there is no beast, but he also knows that he can use the fear of it to control the boys.
Jack blamed the fear on the "beast" but it was just imaginary and he used it to make himself powerful.
Jack wants to create fear and chaos among the boys to strengthen his power and control over them. By making the boys believe the Beast is still alive, Jack can manipulate them into following his orders and relying on him for protection, ultimately furthering his own agenda.
Jack uses propaganda in "Lord of the Flies" by manipulating the other boys through fear and deception. He spreads rumors about the beast to control the group and solidify his power. Jack also uses symbols like the painted faces to inspire fear and loyalty among his followers.
Jack wants to kill. Jack loves the idea of hunting the beast. Ralph continuously says there is no beast. When Samneric report about what the beast looks like, Jack says he will hunt it and asks who wants to join him. Ralph, the more reasonable of the two, says they only have wooden spears. Ralph admits he is scared. He always thought the beast was fake, but now he think there might be a beast. In the end, Ralph and Jack both decide to hunt the beast.
Jack tricks the tribe into believing about the beast to justify the death of Simon and to justify killing Piggy and wanting to kill Ralph. They believe that it hides inside people and wants to murder them all or trick them into doing bad things. The beast is the symbol for fear in this allegorical story.
The Beast and Ralph.
Jack does not blow the conch to call an assembly about the beast because he is becoming more focused on asserting his power in the group and challenging Ralph's leadership. He sees the fear of the beast as an opportunity to undermine Ralph's authority and gain control. Jack's priorities shift from maintaining order and unity to exploiting fear and chaos to his advantage.
Jack tries to downplay the fear of the beast by suggesting that it may not be real or that they can overcome it with resolve and strength. He focuses on instilling confidence in the boys and encouraging them to be bold and fearless to face whatever challenges they may encounter.
In chapter 5 of "Lord of the Flies," Jack dismisses the littleuns' fear and believes they should not be worried about the beast. He asserts that there is no monster on the island and encourages them to join his tribe for protection and safety. Jack sees their fear as a tool to manipulate them into joining his group.
It speaks both to Jack's digression into savagery and Ralph's wish for a save civilization. Jack sees the beast as a chance to vent some of the savagery that he has turned to, and wants to use it as a way to prove himself and the rest of the hunting boys. Ralph sees the beast as a threat to the safety of the boys' world, and thinks that they have to fight it to stay safe.
Initially, Jack and his group viewed the beast as a physical creature to be feared and hunted. However, as the story progresses, their perception of the beast evolves into a symbol of their own inner savagery and fear. They come to see the beast as a means to justify their increasingly violent and chaotic behavior on the island.