From the Epistle to Diognetus
Michael Kruger quotes from an early Christian author to show that some of the earliest Christians affirmed substitutionary atonement and the imputation of Christ’s righteousness. He concludes:
[T]he Epistle to Diognetus shows that the doctrine of the substitutionary atonement, and also the imputation of Christ’s righteousness, are not late inventions, but were present very early in the history of Christianity. Did some Christian groups hold other views of such matters? Sure. But, the continuity between the teachings of this epistle, and the writings of Paul himself (see especially Romans 5), make it evident that the substitutionary atonement/imputation view goes back very early indeed.
From Berkhof’s Systematic Theology
Both the active and passive obedience of Christ are necessary if sinners are to have eternal life (The Reformed Reader).