In chapter 19, what does the slave say about the Dutch religion that upsets Candide?

In chapter 19, Candide meets a black slave missing his left leg and right hand. Candide asks him what happened. The slave explains that when you lose a finger in the mill on the sugar plantation, the custom is to cut off your hand, and if you run away, the practice is to cut off your leg. He has done both, which explains his missing limbs. He points out that the sugar Europeans eat is purchased with such misery. He then says that his parents sold him into slavery to a Christian slaveowner Vanderdendur. At Vanderdendur's Dutch church, he is taught that blacks and whites are all brothers and sisters descended from Adam. The slave feels that such a family treats its member cruelly. When Candide hears this story, he is so shocked that he gives up the philosophy of Optimism:

"Here is an end of the matter. I find myself, after all, obliged to renounce thy Optimism."
"Optimism," said Cacambo, "what is that?"
"Alas!" replied Candide, "it is the obstinacy of maintaining that everything is best when it is worst."

This section, as we can see, continues Voltaire's theme of bitterly criticizing and satirizing religious hypocrisy.

In chapter 19 of Candide, our hero and Cacambo are in Surinam when they come across a black slave in rags lying on the ground. His left leg and right hand are missing, hacked off by his Dutch master as punishment for offending him. The slave's master professes to be a Christian. Indeed, the Dutch converted the slave to Christianity, and at church every Sunday, the slaves are taught that there is no difference between black people and white people; they are equally the children of Adam.
Candide is outraged by the blatant hypocrisy of the Dutch slave traders. They are supposed to be Christians; they claim to believe in racial equality but how they treat their slaves provides evidence to the contrary. They clearly do not practice what they preach. Like so many others, they fail to see the importance of living out their faith and living their life according to your religious values; they just talk about their to make themselves look and feel morally superior.