The pardoner in The Canterbury Tales always addresses the same theme: radix malorum est cupiditas, or “greed is the root of all evil.” All of his preaching is about greed. As he says: "The curse of avarice and cupidity / Is all my sermon, for it frees the pelf. / Out come the pence, and specially for myself" (lines 16-18). In other words, the reason he criticizes his parishioners' greed is to get "pelf," or riches, from them. When he curses greed, money flows more freely into his coffers.
The pardoner also sells relics that the Pope gave to him, and he is willing to provide absolution, or forgiveness of one's sins, in exchange for a shilling. The pardoner openly admits that he is guilty of the covetousness, or greed, against which he preaches. He doesn't care about his parishioners' souls or about starving children, but he says he does care about drinking wine and having a "jolly wench" everywhere he goes. The pardoner is a hypocritical person who knows how to manipulate people to get what he wants.
According to the Pardoner, he always uses the same text for his sermons (in a sermon the preacher is supposed to use some Biblical text -- he's supposed to talk to the congregation about the meaning of the passage that he has selected). The Pardoner says that the text for his sermons is always the famous passage that says that the love of money is the root of all evil (this is from 1 Timothy Chapter 6). So presumably he is talking about how greed is bad.
For I know all by rote that I tell.
My theme is always one, and ever was;
Radix malorum est cupiditas.
This is pretty ironic because he himself is a pretty greedy man and pardoners as a group were known for being pretty greedy.