In Luther’s day, the Christian Church claimed your sins could be forgiven by paying a certain amount of money, a process known as indulgence. This made Luther really mad because in his experience he had really struggled with the sense he could never get right with God. It didn’t matter what he did or forced himself to do, he could not work for or simply buy redemption from his guilt.
He found the solution to this problem in the Bible – in chapter five of Paul’s Letter to the Romans: “Since we are justified by faith, we now have peace with God through our Lord Jesus Christ.” That means that a person can only be saved and live with God in eternal life if Jesus serves as their advocate before God. For Luther it became perfectly clear that Jesus Christ had paid off the guilt or, as Christians say, sin that separates us humans from God.
That is why we humans can come to God – but only by God’s grace. All we have to do is turn to Jesus and believe that he gives us this forgiveness from sin, i.e. have faith in him. The basis for this is the Christian Bible, in which we can read a lot about Jesus. The Christian term for this is gospel, which means good news. Luther’s decisive insight can be summarised quite simply: Only the Bible, only by faith, only by grace, only through Jesus Christ.