What do you think was Irving's purpose in writing "The Devil and Tom Walker"?

I think Washington Irving's main purpose for writing "The Devil and Tom Walker" was to critique and warn of the dangers of greed. In the story, Tom Walker meets a man in the woods who is implied to be the Devil. Tom is in an unhappy marriage and wants nothing more than to be rich. Irving insinuates that Tom eventually agrees to sell his soul to the Devil in exchange for wealth on Earth. For years, Tom makes a lot of money as an usurer, and buys a large house that he barely furnishes "out of parsimony."

As Tom becomes older, he begins to worry about what will happen to him when the Devil comes to make good on their agreement. He turns to religion very loudly but in a very superficial way. He attends church frequently and prays loudly, but continues to charge financially debilitating interest rates when he loans money. Including Tom's turn to religion in the story is Irving's way of critiquing people who are very loud about their religious beliefs but do not actually act in ways that are in keeping with their purported religion.

Ultimately, Tom's day of reckoning does come. When he is accused of making people pay exorbitant interest rates for his own financial gain, Tom says, "The devil take me if I have made a farthing!" At this claim, the Devil appears and does take Tom. After Tom's sudden disappearance, which it can be assumed is his death, people begin to look through his possessions.

On searching his coffers all his bonds and mortgages were found reduced to cinders. In place of gold and silver his iron chest was filled with chips and shavings; two skeletons lay in his stable instead of his half starved horses, and the very next day his great house took fire and was burnt to the ground.

The complete decimation of all of Tom's signs of wealth is Irving's way of critiquing people's obsessions with possessions and spending so much time acquiring material goods when they mean nothing once people die.