It is important to realise that Tom's religious conversion is yet another example of satire. Irving is making a very critical comment about those who try to mask their sin or to conceal it by a show of insincere piety. Let us note that Tom only becomes interested in religion as he grows older, and obviously, religious matters become more of a concern as he has to face the consequences of his tryst with the devil. In addition, note how the narrator describes his conversion:
He became, therefore, all of a sudden, a violent churchgoer. 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 clamour of his Sunday devotion.
So great is his show of piety that other members of the church find themselves suddenly "outstripped" by his violent show of repentance. Clearly, the humorous tone of this quote reveals Tom's insincerity. The thought of heaven being "taken up by force of lungs" is clearly ludicrous, and just reinforces the desperate fear that lies at the heart of Tom's conversion and its insincerity. Thus this conversion does not work because it is false and not based on truth. Tom remains the same kind of sinner that he always was, but his sin becomes all the more abhorrent by the way he tries to cover it up with fraudulent piety.