What is the moral lesson in The Canterbury Tales?
Taken as a whole, I think the Tales reveal how very different people can be, even when they appear similar on the outside. You certainly see examples of both moral and immoral individuals, and often the moral ones are not who you might expect. The Pardoner should be focused on sin and saving souls, but he almost seems to encourage sin in order to raise more money. The Parson, on the other hand, is very moral. So, the job doesn't seem to determine the morality.
However, I agree with the others that each tale has its own moral and clumping them together is not the best way to study them.
The Canterbury Tales is not a single tale, but a collection of them and each has its own moral lesson. To answer your question in a truly helpful manner I would need to know which tale it is that you are referring to.
I would highly recommend looking at the various resources eNotes has on The Canterbury Tales. Specifically, the "Themes" page may be helpful to you as it addresses several reoccurring ideas and morals in the stories.
Also, if you want to let me know specifically what tale it is you are referring to I can be of more help.
Best of luck!
I absolutely agree that each tale is different from the others and that lessons can be taken from each of them. I don't believe that every tale had a moral, honestly. Part of Chaucer's aim was to satirize the church, for example. I think one thing the church's characters' tales reveal is that the church was not what it seemed at times.
There is no overall moral lesson for the tales. Each one has something different to say. That's what makes the Canterbury Tales so lasting. Each generation can find something in the characters and the tales they tell to fit the current situation.