The phrase "simul iustus et peccator" is a Latin phrase that comes out of the Protestant Reformation. Martin Luther used it to describe the state of a believer in Christ. He is simultaneously just and sinful. In Christ, through faith in Him because of His death and resurrection for us, we are simultaneously declared righteous or "right" in God's sight and yet sinful.
This truth, this reality, is not some mere factual declaration. Living in this is exactly what frees us. For it is how the old us is slain in the shadow of the cross, and it is what brings the new us out of the grave. The cross of Christ tells you, "Jesus had to die because of your sins. All of your pretensions of goodness, all your moral improvement projects are for naught. The blood of the Son of God was necessary for you." This is the death knell to all of our spiritual pride, to the old Adam in us... the same old Adam that likes to do bad things or likes to maintain control by trying to be good. It says, "Your efforts are for naught. You are cursed, and this is what it took to save you."
But then it says, "Yet the grace of God has come in this cross, in this crucified Savior, to blot out all your sins and bring you into the throne room of grace." This is where faith is awakened, where the new man is awakened in us. Grace upon grace. God has not cast us off. He has done the impossible. You are the prodigal, the lost son and sinner, and the Father has thrown a party for you. You are justified, accepted, adopted...
The combination of these truths is what undergirds the Christian life and what fuels our change and growth in faith and love. Seeing that the cross declares us "peccator" (sinful), all the aspirations of the old Adam in us with his sin and pretenses and self-sufficient self-improvement programs are exposed as futile, ending in and deserving only death. But seeing that the cross declares us "iustus" (righteous), a new love is kindled in the heart in which we find ourselves allied with God anew. The old is condemned to death and the new is awakened in faith.
Seeing this is a daily need. It is something to live in. It is not something we need just once, and it is through this that we are made new, day by day, as the old Adam is constantly sent back to the grave. As Forde wrote,
"We should make no mistake about it: sin is to be conquered and expelled. But if we see that sin is the total state of standing against the unconditional grace and goodness of God, if sin is our very incredulity, unbelief, mistrust, our insistence on falling back on our self and maintaining control, then it is only through the total grace of God that sin comes under attack, and only through faith in that total grace that sin is defeated. To repeat: sin is not defeated by a repair job, but by dying and being raised new."