Packer:

The Truth about the Reformers to be this They habitually spoke of atonement in the language of legal representation. Christ, they said, personam nostram sustinuit at the bar of God, enduring the penalty of the broken law in man’s place. But they did not distinguish the two possible ways in which the notion could be developed, nor opt for one rather than the other. The transaction as they described it could be thought of after the analogy either of civil or of criminal law. In the first case, Christ would have paid our debt, the second our penalty. In the first case, his righteousness would be imputed to the believer in its effects: to say that Christ’s righteousness is reckoned his would then be merely a way of saying that he receives the benefit of legal immunity and as a result of Christ’s intervention on his behalf. Christ paid the debt; therefore he himself need never pay it. Such a conception, is simple. But if the transaction were interpreted in the second way, as a case of penal substitution, the idea becomes more difficult and the pitfalls more numerous. This way in fact the main strand in the Reformers’ thought. On the first view, the ground of the sinner’s acquittal from liability to punishment (i.e., his justification) is the fact that Christ paid the debt he had incurred. On the second view, the ground of justification is God’s attribution (imputation) of Christ’s obedience and suffering to the guilty sinner. The notion is Biblical, but demands careful statement, which in the early days of Protestant theology it did not always receive. In those days, the two lines of thought existed side by side in the minds of the some men without any awareness of inconsistency. Both served to illustrate the position which it was the Reformers’ supreme concern to demonstrate and defend, that the sinner’s justification is secured, not by his own work, but by his faith in the work Christ did on his behalf. To illustrate this point yet further, they sometimes permitted themselves to speak as if Christ became a sinner in fact at Calvary, and as a man becomes righteous in fact when he believes. During the century which followed, Protestant theologians devoted themselves to the task of systematizing, polishing and defending the Reformers’ teaching. One of the results of these analytical labours was the discovery of inconsistency between these two lines of thought concerning Christ’s satisfaction for sin . Hence arose controversy concerning the nature of imputed righteousness.

James I. Packer, “The Redemption and Restoration of Man in the Thought of Richard Baxter: A Study in Puritan Thought,” (Ph.D diss.,University of Oxford, 1954), 276-277. [Note, what he says, in my opinion, was also as true for many of the later Puritans.]