In 1520, Martin Luther wrote a book called On the Freedom of a Christian. Luther continued discussion on the doctrine of justification by faith, which said that faith alone justifies people so they can have eternal life. Luther said that good works, or good actions that we do, can not get us to heaven. Only faith in Jesus can do that.
Luther said that the Bible is split into two portions: the precepts and the promises of God. He said that the precepts and the promises were the Old Testament and the New Testament, respectively. The purpose of the promises is to show people how sinful and powerless they are, and the purpose of the promises are to show the reader that believing in Christ is the way to fulfill the law. Luther also said that the soul which cleaves to the promises with firm faith will be penetrated by their virtue and be justified, sanctified, and be filled with every good thing. Luther said that this is how a person can be justified by faith in Christ apart from works.
John Calvin, a French Reformer who lived mostly in Switzerland, also wrote a large book named Institutes of the Christian Religion. One of the topics discussed in his book was predestination. Luther taught predestination as well, but he did not emphasize it as much. Luther said that God predestines people to heaven, but did not say that God predestined people for hell, saying that they condemn themselves to hell by their actions. Calvin, on the other hand, said that God predestines people to both heaven and hell.
Calvin also answered those who said predestination makes God into a being who dispenses justice equally. Calvin said that all people are guilty, but God chooses some people to save out of mercy. The reason God doesn’t save everyone is to show He is a just judge. The reason He doesn’t condemn everyone to hell is because of his great mercy.