Mathetes to Diognetus

One of the earliest Christian letters in existence, likely written very shortly after the death of the last apostle, is anonymous, addressed to a certain Diognetus by one who simply calls himself, "Mathetes," that is, a "Disciple". Diognetus is an unbeliever, and hence the letter is primarily evangelistic. So what about the gospel would so early a Christian emphasize, in his endeavors to spread the good news to others? Nothing, of course, but the "Great Exchange"! Consider his following words:

"But when our wickedness had reached its height, and it had clearly been shown that its reward, punishment and death, was impending over us; and when the time had come which God had before appointed for manifesting His own kindness and power, how the one love of God, through exceeding regard for men, did not regard us with hatred, nor thrust us away, nor remember our iniquity against us, but showed great longsuffering, and bore with us, He Himself took on the burden of our iniquities, He gave His 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!"

So where are they who claim that justification only by an imputed, external righteousness is a doctrine new to the Protestant Reformation?