Core to the Reformation was a simple, profound doctrine that Luther embraced that shaped his understanding of the Gospel.  This phrase describes a tension that the Christian finds himself in. It presents us with the paradox of sinful, broken people have also been rescued and redeemed.  We are holy yet sinful, new yet old, saints and sinners.

There’s a latin phrase that typically gets associated with Luther’s teaching:

Simul justus et peccator. 

We are simultaneously justified and sinner.

We are saints and sinners at the same time.

One blog describes the phrase well :

"Perhaps the formula that Luther used that is most famous and most telling at this point is his formula simul justus et peccator. And if any formula summarizes and captures the essence of the Reformation view, it is this little formula. Simul is the word from which we get the English word simultaneously. Or, it means ‘at the same time.’ Justus is the Latin word for just or righteous. And you all know what et is... You remember in the death scene of Caesar after he’s been stabbed by Brutus he says, “Et tu, Brute?” Then fall Caesar. And you too Brutus? It simply means and. Peccator means sinner.

And so with this formula Luther was saying, in our justification we are one and the same time righteous or just, and sinners.” - Ligonnier.org

The Christian life is found in the tension.  Are we sinners or are we saints?  Yes.  We find ourselves simultaneously both sinners and saints at the same time.  While we are identified with Christ, a battle still wages war in us as our old self battles against the new.

“Simultaneously” points to this time between the times—the co-existence of two “times” at the same time: the old age and the new creation are both present realities. - Tullian Tchividjian

This is why Paul in Romans writes, “what I want to do I do not do.”  Because Paul understands that the sinner in him is constantly in tension with the saint in him.  Part of him is guided by the Spirit and looks outward with love for others.  And another part of him is guided by his sinful nature and looks inward at what would be best for himself.

On the one hand we are completely unrighteous.  Romans 3 says, "None is righteous, no, not one; no one understands; no one seeks for God. All have turned aside; together they have become worthless; no one does good, not even one.”

But on the other hand we are completely righteous because of the work of Christ.  Romans 3 also says, "This righteousness is given through faith in Jesus Christ to all who believe. There is no difference between Jew and Gentile, for all have sinned and fall short of the glory of God, and all are justified freely by his grace through the redemption that came by Christ Jesus."

Saint and sinner.  Both at the same time.  The tension between the old you and the new you.  The tension between sin at work and God at work.  The tension between the god you worship and the God you worship.

Embrace the tension.