The quote (below) is from The Epistle to Diognetus 9, translated by Maxwell Staniforth. This text dates from the mid to late 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”.
"He showed how long-suffering He is. He bore with us, and in pity He took our sins upon Himself and gave His own Son as a ransom for us – the Holy for the wicked, the Sinless for sinners, the Just for the unjust, the Incorrupt for the corrupt, the Immortal for the mortal. For was there, indeed, anything except His righteousness that could have availed to cover our sins? In whom could we, in our lawlessness and ungodliness, have been made holy, but in the Son of God alone? O sweet exchange! O unsearchable working! O benefits unhoped for! – that the wickedness of multitudes should thus be hidden in the One holy, and the holiness of One should sanctify the countless wicked!"