In the year 427/428, St. Augustine endeavored to reevaluate his prolific writings in order to correct them of “anything that offends me or might offend others (Augustine, 8) Among his various corrections and explanations, he offered a new insight into his interpretation of the identity of the speaker in Romans 7.14-25. Referencing his book entitled An Explanation of Certain Passages from the Epistle of the Apostle to the Romans, he states:

In this book I said: “However, when he says, ‘We know that the Law is spiritual; but I am carnal, adequately shows that the law can be fulfilled only by spiritual men, the kind that the grace of God transforms,” I certainly did not want this applied personally to the Apostle who was already spiritual, but to the man living “under the “Law” but not yet “under grace.” For prior to this time, in this was I understood these words which, at a later date . . . I reflected upon this more deeply and I saw that his own words can also be understood about he Apostle himself: “We know that the Law is spiritual; but I am carnal
( 96-97)
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The above quote is where Luther got THIS from. . .