"Lordynges," quod he, "in chirches whan I preche,
I peyne me to han an hauteyn speche,
And rynge it out as round as gooth a belle,
For I kan al by rote that I telle.
My theme is alwey oon and evere was -
'Radix malorum est Cupiditas.'
These are the Pardoner's opening words. When he preaches in churches, he cultivates a certain way of speaking, a "hauteyn speche". His voice rings out roundly like a bell does - and he knows everything "by rote" that he says. He only has one theme, and he's only ever had one: "Radix malorum est Cupiditas", or, in English "Greed is the root of all evil".
The problem is that the Pardoner himself is greedy, and he has no desire to help his congregation but instead simply wants to make money.
For myn entente is nat but for to wynne,
And no thyng for correccioun of synne.
His "entente" (intention) is only to "wynne" (make profit) and his intention has nothing to do with the correction of sin. So the Pardoner's morals are entirely separate to those of his sermon - entirely opposite, even. He does not practice what he preaches.
Thus kan I preche agayn that same vice
Which that I use, and that is avarice.
He preaches against the same vice which he himself has.
For though myself be a ful vicious man,
A moral tale yet I you telle kan.
The Pardoner has no morals, and his character is "vicious" (fully vicious!). Yet here's the paradox - he can still ventriloquise a morally instructive story.