In his T4G address, “Did the Fathers Know the Gospel?” (audio forthcoming), Dr Ligon Duncan quoted from The Epistle to Diognetus 9:2–5, a document dated as early as A.D. 117. In the excerpt you can clearly hear an emphasis on justification, imputation, and substitution. Here is the excerpt, taken from Michael W. Holmes’s translation of The Apostolic Fathers [(Baker, 2007), pp 709, 711]:
… when our unrighteousness was fulfilled, and it had been made perfectly clear that its wages—punishment and death—were to be expected, then the season arrived during which God had decided to reveal at last his goodness and power (oh, the surpassing kindness and love of God!). He did not hate us, or reject us, or bear a grudge against us; instead he was patient and forbearing; in his mercy he took upon himself our sins; he himself gave up his own Son as a ransom for us, the holy one for the lawless, the guiltless for the guilty, the just for the unjust, the incorruptible for the corruptible, the immortal for the mortal. For what else but his righteousness could have covered our sins? In whom was it possible for us, the lawless and ungodly, to be justified, except in the Son of God alone? O the sweet exchange, O the incomprehensible work of God, O the unexpected blessings, that the sinfulness of many should be hidden in one righteous person, while the righteousness of one should justify many sinners!
On a related note, two years back I sat down with Dr. Duncan to discuss the church fathers. The fruit of that conversation was the audio interview “Patristics for Busy Pastors.”