Writing in the TLS David Runciman explains:
But between the political parts [of Leviathan] – the first two sections and the final one – come parts three and four, which are concerned with religion. This bit of the book, which makes up nearly half the total, is entirely uncompromising. Hobbes uses it to demolish all those claims to religious authority that he despised, whether coming from Presbyterians or Catholics, bishops or Bible-bashers. He deploys a combination of selective biblical citation and his own materialist philosophy to lay into every absurd religious idea he can find: demons, fairies, the holy spirit, the life everlasting, the immortal soul. Life, for Hobbes, means motion, and when motion ceases, there is only death.
This all-out assault on religious superstition and stupidity is what makes Leviathan a very different book from De Cive, which contains no equivalent. Hobbes’s urgency in 1649–50 derived in large part from his fear that a new political order might provide a fresh opportunity for the peddlers of religious charlatanry to get their hooks into the state.