Appearances
The novella is based on the idea that what we see does not always reflect the truth. Every character in the book plays a define role, dons different masks in public, and behave in a certain manner when they try to achieve something – usually at the expense of other characters. As Shamela is a satire, Fielding wants the reader to question the laudatory representation of characters in Pamela, offering alternative perspectives on the virtuous Pamela, the servants, and Squire B.
Chastity and Virtue
In Pamela, chastity is something every young girl should possess, and if she is not chaste and virtuous her reputation is sullied and she may have difficulty securing a husband. But chastity is a slippery concept and something that is easily faked. In Shamela, chastity is a commodity and virtue is used as a mask to hide a rotten core. Shamela has no trouble pretending to be virtuous, and the novel suggests that people should not have been so quick to praise Pamela, for her virtue was suspect as well.
Manipulation
Most characters in Shamela are liars and manipulators, seeking to bend events to their will in order to secure or advance their standing. Servants help Shamela hoping to curry favor with the putative mistress of the house; the Squire punishes and rewards Shamela; Parson Williams deliberately misinterprets his religion in order to keep getting what he wants; and Shamela practices deceitful behavior, including pretending to drown herself, in order to ensnare the Squire and simultaneously keep sleeping with Parson Williams. In Pamela the bad behavior, while muted, is generally successful, but in Shamela Fielding has his characters get their comeuppance in order to send a message that vice should not, and will not, be rewarded.
Religion
One of Parson Oliver's most damning complaints about Pamela, revealed in the "true" documents of Shamela, is that Parson Williams is a self-serving, hypocritical, and meddling clergyman who does not deserve to be called a man of God. Certainly Fielding drew him in a slightly unrealistic fashion, as the novella is a satire, but he still functions as a commentary on the problematic nature of religion, particularly Methodism.
Corrupting Influence of Literature
Through the character of Parson Oliver, Fielding articulates prevailing contemporary ideas surrounding the corrupting influence of literature, particularly sentimental fiction like Pamela. Such work can give young people the wrong idea, he maintains: young gentlemen may think it is okay to pursue a chambermaid, and a chambermaid might think it is okay to be pursued. Vice instead of virtue is rewarded, and the social order is upset. A text like Shamela, then, is not just a satire but also a corrective to the presumed dangers lurking in the pages of Pamela.
Politics
This is a subtler theme, but it is there in the penumbras of the text—especially for contemporary readers. Squire Booby has Parliamentary ambitions and Parson Williams is trying to consolidate his power in his curate, but perhaps more importantly, contemporary readers might be able to pick up on Fielding's comparison of the Squire to the King and Shamela to his manipulative prime minister, Sir Robert Walpole. Fielding suggests that Shamela's ability to beguile Squire Booby and its concomitant destabilizing of the domestic social order is akin to Walpole's machinations and the concomitant destabilizing of the national social and political order.
Social Class
To Fielding and many of his contemporary readers it is deeply problematic that Pamela, a woman from a lower social class, will marry a man like the Squire. Thus, in Shamela, the original text's problematic obfuscation of class distinctions is now ludicrously and glaringly wrong: Shamela is crass, manipulative, and immoral, and while the Squire is no paragon of morality, he is clearly being wronged by her and is elevating a meretricious woman to a position of a great lady.