Martin Luther: He is talking about how he use to think god was an angry being and that he wanted to punish. But he later realized that he is not angry or mad but more like a father looking over everyone trying to help us. And just having faith in him will open the gates to heaven.
Basically Johann is saying that god is harsh and will punish you for your sins. All thought he may not believe it, he is using it for his campaign for the sale of indulgences. While Martin is saying that god loves us all and he is here to help us all and we should just have faith in him not buy indulgences or do good deeds.