The Reformation’s emphasis on faith alone was the result of Luther’s tortured struggles to resolve the issue of how a fallen sinner may be saved. “My situation was that, although an impeccable monk, I stood before God as a sinner troubled in conscience, and had no confidence that my character would satisfy Him. Night and day I pondered,” Luther said. The breakthrough came when Luther was given insight into Romans 1:17: “For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith.” Luther later wrote: “Then I grasped that the justice of God is the righteousness by which, through grace and sheer mercy, He justifies us through faith. Immediately I felt myself to have gone through open doors into paradise.”
2 Corinthians 5:14–21 (ESV)
14 For the love of Christ controls us, because we have concluded this: that one has died for all, therefore all have died; 15 and he died for all, that those who live might no longer live for themselves but for him who for their sake died and was raised.
16 From now on, therefore, we regard no one according to the flesh. Even though we once regarded Christ according to the flesh, we regard him thus no longer. 17 Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come. 18 All this is from God, who through Christ reconciled us to himself and gave us the ministry of reconciliation; 19 that is, in Christ God was reconciling the world to himself, not counting their trespasses against them, and entrusting to us the message of reconciliation. 20 Therefore, we are ambassadors for Christ, God making his appeal through us. We implore you on behalf of Christ, be reconciled to God. 21 For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.