John Beckman passes along this good early quote (about AD 130) on Christ’s righteousness in justification:
He himself took on Him the burden of our iniquities, He gave His own Son as a ransom for us, the holy One for transgressors, the blameless One for the wicked, the righteous One for the unrighteous, the incorruptible One for the corruptible, the immortal One for them that are mortal. For what other thing was capable of covering our sins than His righteousness? By what other one was it possible that we, the wicked and ungodly, could be justified, than by the only Son of God? O sweet exchange! O unsearchable operation! O benefits surpassing all expectation! that the wickedness of many should be hid in a single righteous One, and that the righteousness of One should justify many transgressors!
(ANF1, Epistle to Diognetus, chapt. 9, p. 28)