When our wickedness had reached its height, and it had been clearly 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 long-suffering, and bore with us…

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!

The quote is from The Epistle to Diognetus 9, translated by Roberts-Donaldson. This text dates from early to mid 2nd century AD. It is an early indication that the doctrines of substitutionary atonement and double imputation were not first the product of the Protestant Reformation, but were held dear by the earliest generations of Christians. The author is unknown – he refers to himself simply as a mathetes “disciple”.