Simul justus et peccator
(Latin simul, “simultaneous” + Latin justus, “righteous” + Latin et, “and” + Latin peccator, “sinner”)
Roman Catholic theology maintains that baptism washes away original sin. However, “concupiscence” remains as an inclination to sin, which is not sin unless actualized. Luther and the Reformers, following Augustine, insisted that what was called “concupiscence” was actually sin. While not denying the validity of baptism, Luther maintains that the inclination to sin is truly sin.
“Simul justus et peccator” simply means that a Christian’s righteousness or justification imputed in baptism is a gift of Christ, freely given despite the sinner’s condition. The doctrine of “simul justus” is not an excuse for lawlessness, or a license for continued sinful conduct; rather, properly understood, it comforts the person who truly wishes to be free from sin and is aware of the inner struggle within him. Romans 7 is the key biblical passage for understanding this doctrine.
Luther also does not deny that the Christian may ever “improve” in his conduct. Instead, he wishes to keep Christians from either relying upon or despairing because of their own conduct or attitude.