It usually doesn't take long while talking about human nature with a Lutheran pastor that the term simul iustus et peccator comes up. This term means that a Christian is at the same time completely justified before God through faith in Jesus, and yet struggles against sin because God has not changed the individual's nature.
God's Word clearly teaches every human is born a sinner (Genesis 8:21 and Psalm 51:5). This truth is proven because people die as a result of sin (Romans 6:23a). Sin affects people of every age, class, and race. No one can avoid it. Every day we struggle against sin and its effects in life.
Jesus died on the cross, taking the sins of all people upon Himself, so that God no longer sees the sin, but only Jesus' holiness. For this reason we are declared forgiven, holy and just in His sight. Christians believe we have received the grace of God (Eph. 2:8-9) and now have eternal life (Romans 6:23b). This is God's promise and is not dependent upon anything the Christian does (Ephesians 2).
But we have a problem. Christians know we are born in sin. We know that Jesus died taking away all our sin and we completely trust He has given us eternal life. We know better, but still do things we shouldn't and don't do the things we should (Romans 7:18-20). Our life is in tension, but we hold on to the promise that while we still struggle to let our new nature reign, we have the promise "There is therefore now no condemnation for those who are in Christ Jesus" (Romans 8:1). The tension continues throughout life.
Our Lutheran ancestors clearly summarized these teachings in the Book of Concord. Luther explained the Third Article of the Creed by saying that we are unable to come to Jesus on our own. A barrier of sin keeps us from God. Only the Holy Spirit using the Word and Sacraments can bridge that gap and bring God's judgment that in Christ we are just and holy.
Jesus taught all His disciples to pray "forgive us our trespasses" because He knew they would sin even after faith. Martin Luther explains "Although we have God's Word and believe … nevertheless we are not without sin. We still stumble daily and transgress…."
Our baptism assists us in this struggle between the old and new natures. Luther encourages us to use our baptism to drown daily the old sinful nature and to let the new nature in Christ arise and live freely. It is a daily process of seeking to be more Christ-like with the help of the Holy Spirit.
God gave the Ten Commandments to His people after saving them from slavery. Luther gave meaning to these commandments in both a negative (what we should avoid) and a positive way, (what we now want to do because we have been delivered from slavery—physical or spiritual). These commandments are in effect until Christ ushers in the heavenly kingdom when the struggle against sin is ended.
The importance of this teaching becomes clear daily in life. It is easy for Christians to wonder why we still struggle with the same sins we had before knowing Jesus as Saviour and after being declared holy. This doctrine assures us we are living a normal Christian life.
As Christians struggle with sin we can wonder if we will ever be released from the curse of the sin we endure. As forgiven sinners we have the promise of eternal life in Christ right now. Christ has redeemed us! The forgiveness sometimes seems hidden, but God's promise of forgiveness to all who trust in Him is good no matter what the struggle.
This teaching also keeps Christians from false security thinking we no longer sin. If that were the case Christ is no longer needed. We constantly need to pray "forgive us our trespasses" and to put on the full armour of God to fight against the attacks of sin, society and self. The war was won on Good Friday, but we still face battles. We have the victory, but Satan doesn't quit.
Lutheranism contributes to Christian theology the ability to live in the tensions the Bible creates for us. The Bible clearly teaches we are both declared fully just before God, without sin (Jeremiah 31:34 "I … will remember their sins no more.") and still stuck in the struggle against the old human nature (Romans 7:24 24 "Who will rescue me from this body of death?). Lutherans let both teachings stand knowing that the ultimate victory is won and given in Christ.