Why was martin luther excommunicated and declared heretic?

Martin Luther was excommunicated becuase he was going against the Roman Catholic church. He was saying that you didn't need to pay indulgences to the Roman Catholic church just to get into heaven. All you had to do was believe that Jesus christ died on the cross to save our sins. So when he went against the Roman Catholic church, the pope was mad becuase if people started believing him, they would lose money. I hope this helped you out!