Let's first deal with the structure of the present work. Book 5 begins strangely. We have been building up from Book 1 in anticipation of a discussion of Marcion's Antitheses. Mention is made of the work throughout Books 1, 2, 3 and 4. Strangely though mention of the Antitheses disappears in Book 5. Book 5 begins with an even stranger discussion of 'the person of Paul' which seems entirely divorced from what precedes it in Book 4. The Marcionite Paul is unknown. Tertullian confesses he is a 'new convert' to Christianity and then a long digression with no mention of the origin or form of the Marcionite canon.
Instead the Galatians section begins with the following words - again wholly attached from anything which precedes it in Books 1 - 4:
Now remember there is NO discussion of anything that precedes these words. There is just the bald statement that Galatians is the most decisive against Judaism out of the blue. Of course this is also debatable. Romans has many antinomian statements that the heretics loved. But let's look at the Romans section by comparison which form chapter 13 of the 21 chapter work:The epistle which we also allow to be the most decisive against Judaism, is that wherein the apostle instructs the Galatians. For the abolition of the ancient law we fully admit, and hold that it actually proceeds from the dispensation of the Creator,----a point which we have already often treated in the course of our discussion, when we showed that the innovation was foretold by the prophets of our God.46 Now, if the Creator indeed promised that "the ancient things should pass away,"47 to be superseded by a new course of things which should arise, whilst Christ marks the period of the separation when He says, "The law and the prophets were until John"48 ----thus making the Baptist the limit between the two dispensations of the old things then terminating----and the new things then beginning, the apostle cannot of course do otherwise, (coming as he does) in Christ, who was revealed after John, than invalidate "the old things" and confirm "the new," and yet promote thereby the faith of no other god than the Creator, at whose instance49 it was foretold that the ancient things should pass away. [2] Therefore both the abrogation of the law and the establishment of the gospel help my argument even in this epistle, wherein they both have reference to the fond assumption of the Galatians, which led them to suppose that faith in Christ (the Creator's Christ, of course) was obligatory, but without annulling the law, because it still appeared to them a thing incredible that the law should be set aside by its own author.
I have always wondered why Tertullian exclaims that the little opus is ending in the middle of Book 5? Book 5 can't be the 'little opus.' The only opus is the work itself - i.e. all of the book. As such doesn't it make more sense to assume that the original work began with Romans (as our canon has it) and later one of the many changes wrought by the 'apostate' (cf. AM 1.1) was the reordering of the letters according to the eastern ordering of the Pauline letters (cf. Ephraim's canon) viz. "[t]herefore, there are indications from both Ephrem and the Syriac canon list that the Old Syriac had the letters of Paul in the order Galatians, Corinthians, Romans" https://www.google.com/search?hl=en&tbm ... qxKTMirETMAs this little opus comes to an end (quanto opusculum profligatur), I must treat but briefly the points which still occur, whilst those which have so often turned up must be put aside. I regret still to have to contend about the law----after I have so often proved that its replacement (by the gospel)595 affords no argument for another god, predicted as it was indeed in Christ, and in the Creator's own plans ordained for His Christ. (But I must revert to that discussion) so far as (the apostle leads me, for) this very epistle looks very much as if it abrogated597 the law. [2] We have, however, often shown before now that God is declared by the apostle to be a Judge; and that in the Judge is implied an Avenger; area in the Avenger, the Creator. And so in the passage where he says: "I am not ashamed of the gospel (of Christ): for it is the power of god unto salvation to every one that believeth; to the Jew first, and also to the Greek; for therein is the righteousness of God revealed from faith to faith,"598 he undoubtedly ascribes both the gospel and salvation to Him whom (in accordance with our heretic's own distinction) I have called the just God, not the good one. It is He who removes (men) from confidence in the law to faith in the gospel----that is to say,599 His own law and His own gospel. When, again, he declares that "the wrath (of God) is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness,"600 [3] (I ask) the wrath of what God? Of the Creator certainly. The truth, therefore, will be His, whose is also the wrath, which has to be revealed to avenge the truth. Likewise, when adding, "We are sure that the judgment of God is according to truth,"601 he both vindicated that wrath from which comes this judgment for the truth, and at the same time afforded another proof that the truth emanates from the same God whose wrath he attested, by witnessing to His judgment. Marcion's averment is quite a different matter, that602 the Creator in anger avenges Himself on the truth of the rival god which had been detained in unrighteousness. [4] But what serious gaps Marcion has made in this epistle especially, by withdrawing whole passages at his will, will be clear from the unmutilated text of our own copy.603 It is enough for my purpose to accept in evidence of its truth what he has seen fit to leave unerased, strange instances as they are also of his negligence and blindness. If, then, God will judge the secrets of men----both of those who have sinned in the law, and of those who have sinned without law (inasmuch as they who know not the law yet do by nature the things contained in the law)604 ----surely the God who shall judge is He to whom belong both the law, and that nature which is the rule605 to them who know not the law. But how will He conduct this judgment? [5] "According to my gospel," says (the apostle), "by (Jesus) Christ."606 So that both the gospel and Christ must be His, to whom appertain the law and the nature which are to be vindicated by the gospel and Christ----even at that judgment of God which, as he previously said, was to be according to truth.607 The wrath, therefore, which is to vindicate truth, can only be revealed from heaven by the God of wrath;608 so that this sentence, which is quite in accordance with that previous one wherein the judgment is declared to be the Creator's,609 cannot possibly be ascribed to another god who is not a judge, and is incapable of wrath. It is only consistent in Him amongst whose attributes are found the judgment and the wrath of which I am speaking, and to whom of necessity must also appertain the media whereby these attributes are to be carried into effect. even the gospel and Christ. [6] Hence his invective against the transgressors of the law, who teach that men should not steal, and yet practise theft themselves.610 (This invective he utters) in perfect homage611 to the law of God, not as if he meant to ten sure the Creator Himself with having commanded612 a fraud to be practised against the Egyptians to get their gold and silver at the very time when He was forbidding men to steal,613 ----adopting such methods as they are apt (shamelessly) to charge upon Him in other particulars also. Are we then to suppose614 that the apostle abstained through fear from openly calumniating God, from whom notwithstanding He did not hesitate to withdraw men? [7] Well, but he had gone so far in his censure of the Jews, as to point against them the denunciation of the prophet, "Through you the name of God is blasphemed (among the Gentiles)."615 But how absurd, that he should himself blaspheme Him for blaspheming whom he upbraids them as evil-doers! He prefers even circumcision of heart to neglect of it in the flesh. Now it is quite within the purpose of the God of the law that circumcision should be that of the heart, not in the flesh; in the spirit, and not in the letter.616 Since this is the circumcision recommended by Jeremiah: "Circumcise (yourselves to the Lord, and take away) the foreskins of your heart; "617 and even of Moses: "Circumcise, therefore, the hardness of your heart,"618 ----the Spirit which circumcises the heart will proceed from Him who prescribed the letter also which clips619 the flesh; and "the Jew which is one inwardly" will be a subject of the self-same God as he also is who is "a Jew outwardly; "620 because the apostle would have preferred not to have mentioned a Jew at all, unless he were a servant of the God of the Jews. [8] It was once621 the law; now it is "the righteousness of God which is by the faith of (Jesus) Christ."622 What means this distinction? Has your god been subserving the interests of the Creator's dispensation, by affording time to Him and to His law? Is the "Now" in the hands of Him to whom belonged the "Then"? Surely, then, the law was His, whose is now the righteousness of God. It is a distinction of dispensations, not of gods. [9] He enjoins those who are justified by faith in Christ and not by the law to have peace with God.623 With what God? Him whose enemies we have never, in any dispensation,624 been? Or Him against whom we have rebelled, both in relation to His written law and His law of nature? Now, as peace is only possible towards Him with whom there once was war, we shall be both justified by Him, and to Him also will belong the Christ, in whom we are justified by faith, and through whom alone God's625 enemies can ever be reduced to peace. [10] "Moreover," says he, "the law entered, that the offence might abound."626 And wherefore this? "In order," he says, "that (where sin abounded), grace might much more abound."627 Whose grace, if not of that God from whom also came the law? Unless it be, forsooth, that628 the Creator intercalated His law for the mere purpose of629 producing some employment for the grace of a rival god, an enemy to Himself (I had almost said, a god unknown to Him), "that as sin had" in His own dispensation630 "reigned unto death, even so might grace reign through righteousness unto (eternal) life by Jesus Christ,"631 His own antagonist! [11] For this (I suppose it was, that) the law of the Creator had "concluded all under sin,"632 and had brought in "all the world as guilty (before God)," and had "stopped every mouth,"633 so that none could glory through it, in order that grace might be maintained to the glory of the Christ, not of the Creator, but of Marcion! [12] I may here anticipate a remark about the substance of Christ, in the prospect of a question which will now turn up. For he says that "we are dead to the law."634 It may be contended that Christ's body is indeed a body, but not exactly635 flesh. Now, whatever may be the substance, since he mentions "the body of Christ,"636 whom he immediately after states to have been "raised from the dead,"637 none other body can be understood than that of the flesh,638 in respect of which the law was called (the law) of death.639 [13] But, behold, he bears testimony to the law, and excuses it on the ground of sin: "What shall we say, therefore? Is the law sin? God forbid."640 Fie on you, Marcion. "God forbid!" (See how) the apostle recoils from all impeachment of the law. I, however, have no acquaintance with sin except through the law.641 But how high an encomium of the law (do we obtain) from this fact, that by it there comes to light the latent presence of sin!642 [14] It was not the law, therefore, which led me astray, but "sin, taking occasion by the commandment."643 Why then do you, (O Marcion, ) impute to the God of the law what His apostle dares not impute even to the law itself? Nay, he adds a climax: "The law is holy, and its commandment just and good."644 [15] Now if he thus reverences the Creator's law, I am at a loss to know how he can destroy the Creator Himself. Who can draw a distinction, and say that there are two gods, one just and the other good, when He ought to be believed to be both one and the other, whose commandment is both "just and good? "Then, again, when affirming the law to be "spiritual"645 he thereby implies that it is prophetic, and that it is figurative. Now from even this circumstance I am bound to conclude that Christ was predicted by the law but figuratively, so that indeed He could not be recognised by all the Jews.
The point of course is that not only does the author speak of the 'little opus' coming to an end but the cursory mention of things in the epistles. This is true for ALL sections not just Romans and what follows. The discussion of Luke is much more thorough. Hence the statement makes more sense in that context.