What are the themes in Geoffrey Chaucer's The Canterbury Tales?

Geoffrey Chaucer's The Canterbury Tales include so many characters and stories that it is difficult to narrow the entire work down to just a couple of themes—you could probably assign a different theme to every character and every character's tale, which would leave the reader with about 50 different themes.

It is, however, possible to broaden the scope a bit and consider two very general central messages that Chaucer is working to get across.

If we look at characters like the Pardoner, Summoner, Friar, and Monk, we have to consider Chaucer's theme to be satirical—he is criticizing the abuse of power, particularly as it pertains to the church. He says of the greedy Pardoner:

But best of all he sang an Offertory,

For well he knew that when that song was sung

He'd have to preach and tune his honey-tongue

An (well he could) win silver from the crowd.

We could state such a theme this way: People who are granted power over the helpless or less fortunate are sometimes liable to abuse that power, to the detriment of those they are supposed to serve.

Chaucer does not focus only on such corrupt characters. He also lovingly characterizes the Parson:

He was a shepherd and no mercenary.

Holy and virtuous he was, but then

Never contemptuous of sinful men,

Never disdainful, never too proud or fine,

But was discreet in teaching and benign.

The Parson treats his “sheep” as a parson should; he does not try to take advantage of them. He is even kind and patient with sinners.

His brother, the Plowman, is similarly good:

He would help the poor

for love of Christ and never take a penny

If he could help it,

The Parson and the Plowman exemplify the following theme:

Those who truly live according to God's will do good for those who need it most.

These are not the only possible themes in The Canterbury Tales. There are others, based on what Chaucer wrote about the different characters. There are also multiple ways of stating the themes I have given above. As long as your theme makes a statement about human nature that can be supported with evidence from the text, it should be a good one.