One major target of Irving’s satire in the story is the religious hypocrisy of Tom Walker, and by implication the early Puritans of New England more broadly.
He thought with regret of the bargain he had made with his black friend, and set his wits to work to cheat him out of the conditions. He became, therefore, all of a sudden, a violent church-goer. He prayed loudly and strenuously, as if heaven were to be taken by force of lungs. Indeed, one might always tell when he had sinned most during the week by the clamor of his Sunday devotion [...] Tom was as rigid in religious as in money matters; he was a stern supervisor and censurer of his neighbors, and seemed to think every sin entered up to their account became a credit on his own side of the page. He even talked of the expediency of reviving the persecution of Quakers and Anabaptists.
Tom’s turn to religion is cynically motivated by his desire to cheat the devil rather than any sense of genuine faith, and his sins and religious zeal increase in proportion to each other. What he lacks in morality he attempts to compensate for in volume, praying "loudly and strenuously," as if the physical act of prayer matters more than true belief. The highest expression of Tom’s “faith,” admired by his fellow church-goers, is to call for persecution against religious minorities such as Quakers and Anabaptists. The language of piety is used ironically by Irving to underscore the craven, greedy, unsympathetic character of Tom, but by association he also satirizes the broader religious community that could regard such a man as morally upright.