simul iustus et peccator:
at once righteous and a sinner; Luther’s characterization of the believer justified by grace through faith. Since faith, not works, is the ground of our justification (iustificatio, q.v.), and since justification is not an 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 as measured according to his own merits.
Richard A. Muller, A Dictionary of Latin and Greek Theological Terms: Drawn Principally From Protestant Scholastic Theology (Grand Rapids: Baker, 1985).