Pope Francis isn't mincing words in his latest denouncement of those who exploit the poor.

The pontiff was in his element during a passionate sermon at the Vatican on Thursday, berating rich employers who hire and fire poor people with callous disregard for their workers' wellbeing, and who refuse to give their employees health care, pensions, or vacations.

He called people who take advantage of the poor in this way "true bloodsuckers" who "live by spilling the blood of the people who they make slaves of labor."

Vatican Pool via Getty Images Pope Francis has often critiqued the "idolatry of money."

Although some may think slavery no longer exists, Francis insisted that it is still a problem around the world.

"It’s true, people no longer go to Africa to capture [slaves] in order to sell them in America, no. But it is in our cities. And there are these traffickers, these people who treat the working people without justice," Francis said in his sermon. "This is a mortal sin. And this demands a great deal of penance, a great deal of restitution, in order to be converted from this sin."

In particular, Francis recalled the story of a young girl who told him that she worked for 11 hours a day for 650 euro a month, all under the table. Her employers reminded her that she could leave if she couldn't handle the terms of the job -- there were plenty of others like her who would do the work instead.

Paraphrasing the Bible passage for the day, taken from James 5, the pope said: “The blood of all these people that you have sucked [and on which] you have lived, is a cry to the Lord, it is a cry of justice."

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In a similar vein, Francis also slammed the “theology of prosperity,” which he said is the belief that “God shows you that you are just if He give you great riches.”

Some critics have said that the prosperity gospel preys on the poor who are encouraged to donate money to churches with the promise that the investment will result in an abundance of wealth or miraculous healings later on.

Pope Francis insisted that people who believe in this theology have the wrong idea because it encourages them to become too attached to material goods.

"You cannot serve both God and riches,” Francis said during the sermon.

Pope Francis has often spoken up about the "idolatry of money" and the evils of economic systems that treats people like commodities.

In the end, Francis left his listeners with a poignant metaphor about the importance of helping others:

“A glass of water in the name of Christ is more important than all the riches accumulated through the exploitation of the people.”