Simul Justus et peccator - at once righteous and a sinner.

We have Luther to thank for this expression!

Since the believer is justified by grace through faith, and not works, and since justification is not a infusion of righteousness that makes a sinner righteous 'in and of himself,' the sinner is both righteous in God's sight because of Christ and a sinner measured according to his own merits.
(Thanks to Richard A Muller - Dictionary of Latin and Greek Theological Terms, p 283.)