"However, to the one who does not work but trusts God who justifies the ungodly, their faith is credited as righteousness." Romans 4:5 NIV
Simul Iustus et Peccator is what the Bible calls "justification". When someone puts their faith in Jesus Christ, meaning they are relying on what He did in living a perfect life, dying a substitionary, sacrificial death, and being raised from the grave (1 Corinthians 15:1-6) for forgiveness of sin and the promise of eternal life, they are declared not guilty and righteous by God. The key is we are declared righteous while still in the sinning state. That is the scandal of the gospel.
Several years ago I was asked by a close friend of mine to share some scriptures with his father who was literally on his death bed dying of lung cancer. The first time I went to meet with Jack I quickly realized he was a man full of questions. Blind faith was not an option for him. Like many other people he was skeptical. He was a thinker and could not reconcile faith and science, and many other issues that blocked him from having faith. That particular day I gave him my best apologetics, my best reasoning, my best theological and philosophical arguments for faith in Christ. He did not get it. I felt like I was talking to a brick wall.
A few days later, my friend Barry told me that Jack wanted to meet with me again. I was a little shocked to be honest. So hesitantly I went back to his house. I really did not know what to say. So I asked the Lord what scriptures he wanted me to read to Jack. I felt led to read Romans 5:1-2:
"Therefore, since we have been justified through faith, we have peace with God through our Lord Jesus Christ, through whom we have gained access by faith into this grace in which we now stand..."
After I read that passage, Jack said: "I have a always believed there had to be a God and a Creator, but I never really knew what to do with this Jesus fellow. But according to what you just read I can only come to God through Jesus." I said: "yes Jack that is what it means. Do you want to come to God through Jesus?" He said yes. So we prayed and he came into a relationship with God through faith in Jesus. A few days later he passed away. At his funeral I shared that story with his family and friends that had gathered to grieve his passing. Most of them looked confused because they knew Jack as a man who was not "religious". I shared about the thief on the cross who asked Jesus to let him into heaven when he died (Luke 24:42-43). Jesus promised him that his request that was filled faith would be granted. The thief lived as a sinner yet died justified. So did Jack. So will I. So can you if you put your faith in Jesus Christ.