In the third lecture/chapter, Of the Fathers and Scholastic Divines, Buchanan deals with the common Roman Catholic charge that "Justification by grace through faith" was a novelty of the Reformers.
However, Buchanan makes clear what the question is not.
The question, therefore, is not,--Whether all the Fathers taught the doctrine of Justification in its original purity, nor even whether any one of the Fathers was entirely exempt from the corruptions which were gradually growing up in the church; but simply, whether the doctrine of Justification by grace, through faith in the merits of Christ, may not be traced in the writings of some witnesses for the truth, along the whole line of the church's history...[page 78]He asserts on page 80
It is simply to prove a matter of fact, in opposition to an erroneous assertion,--the fact, namely, that the Protestant doctrine of Justification was not a 'novelty' introduced for the first time by Luther and Calvin,--that it was held and taught, more or less explicitly, by some writers in every successive age...On page 90, Buchanan deals with the term "merits" and the misunderstanding that grew over the centuries. He offers a definition of how it was used in Augustine's day.
In the general sense, as denoting the obtaining or procuring of something, it was said that we might merit Christ, or merit the Spirit, or merit eternal life; not that we could deserve any one of these inestimable gifts, or that they could ever become due to us in justice,--for this is inconceivable,--but simply that they might thus be procured and enjoyed.On the next page, 91, Buchanan then deals with Augustine's use of Justification as "wrought both on the state and character of a sinner". It is true that the church has always taught that God both imputes righteousness and infuses righteousness, but the question is not that God does both but how they are related to one another. As Buchanan rightly argues, one is based upon the other. Sanctification flows out from Justification, and not the reverse.
Although there are several pages of citations from several church Fathers, I will end with Buchanan's citation of St. Athanasius.
"Not by these," i.e. by works, says Athanasius, "but by faith, a man is justified as was Abraham."..."In no other manner can there be redemption and grace to Israel and to the Gentiles, except the original sin, which through Adam passed unto all, be loosed. But this, says he (the Apostle) , can be blotted out through no other than through the Son of God."..."It is necessary, therefore, to believe the Holy Scriptures,--to confess Him who is the First-Fruit of us,...to be struck with wonder at the great dispensation,--to fear not the curse which is from the Law, for 'Christ hath redeemed us from the curse of the Law.' Hence the full accomplishment of the Law, which was made through the First-Fruit, is imputed to the whole mass." [page 94]