What does Simul Iustus et Peccator mean?

In Reformed Theology we enjoy this Latin phrase. This phrase is exclusively for Gods elect….

“In Luther’s language, we recognize ourselves—each one—to be simul iustus et peccator and semper iustus et peccator. So when we disobey Jesus’ commands, whether by failing to do what he requires or by doing what he prohibits, we do not despair. We recognize that sinless perfection is not attainable in a fallen world; that we have been redeemed but not yet fully liberated from our fallen condition. At the same time, we grieve over sin and take action accordingly: we return to the gospel in the beatitudes; we ask the heavenly Father to forgive our debts and to hasten the end of spiritual warfare; and we turn from our sins and submit anew to God’s rule—and to striving toward an elusive and seemingly unattainable goal.”

J. Knox Chamblin, Matthew: A Mentor Commentary, Mentor Commentaries (Ross-shire, Great Britain: Mentor, 2010), 309.