In Mark Twain's classic, The Adventures of Huckleberry Finn, satire abounds. As he humorously criticizes religion, Twain often focuses on the hypocrisy of the "good Christians." In one instance, Huck relates how the Widow Douglas coerces him into going to church, she teaches him Bible verses, and chastises him severely for smoking even though, as Huck relates,
...she took snuff, too; of course, that was all right because she done it herself.
In another instance, when Huck stays with the Grangerfords, the men attend church, but bring their guns, symbolic, of course, of violence and murder.
Further in the novel, the Dauphin easily deceives the religious crowd of mourners in Chapter XX. After the preacher cries out,
Oh, come to the mourners' bench! come, black with sin! (amen!) come, sick and sore! (amen!)...
With hilarious irony, the king responds to the call and asks the preacer if he can be allowed to speak. Then, he fabricates a story of how he has been a pirate, but now he is a changed man thanks to a preacher in a tent in Pokeville, "the truest friend a pirate ever had!"
When the king feigns tears, "so did everybody." The so easily emotionally swayed crowd cries. Then someone "sings out,'Take up a collection!'" And, another encourages with "Let him pass the hat around!" Thus, in this instance, Twain satires the unthinking and religious fools that are so easily swayed and duped by one of the greatest hypocrites of the novel.
I think religion is always an easy target for satire because of what it tries to be. Religion (at least Christianity) tries to claim that its adherents know the way to salvation. It tries to claim that it knows what is moral. The reason it's easy to satire is because it does this even though all of its adherents are (by definition) sinners who will do bad even as they claim to know what is right and to love what is right.
To me, the clearest example of this kind of satire comes in how religious the Phelps family is, even as they commit what is surely a sin by trying to help keep Jim in slavery.
Even before the present day disgraces of Catholic priests and their affairs with young boys and evangelists' sordid affairs of all kind, the church and religion in general has always been an easy target for satire. The idea of Huck Finn in church could lead to nothing less than comic moments. When Tom Sawyer attends church in The Adventures of Tom Sawyer, Twain presents one of the novel's best moments. Tom trades multiple treasures for the chance to meet Judge Thatcher--all for the hope of impressing his new love, Becky.