I have previously mentioned Joshua Moon’s dissertation “Jeremiah’s New Covenant: An Augustinian Reading.” I do not agree with everything he has to say, but I highly recommend reading it (PDF) as he explains Augustine’s view of Jeremiah 31, as well as how that Augustinian reading was held down through church history up until the Reformation. Below is an excerpt (66-74 PDF) of his account of Aquinas’ Augustinian reading of Jeremiah 31 (see my previous Aquinas’ Retroactive New Covenant).
In summary form the lex nova for Thomas is the grace of God through the Holy Spirit.64 Thomas identifies the lex nova with the Law of the novum testamentum (‘lex nova est lex novi testamenti’), and defines the new law as the grace of the Holy Spirit:
‘Each thing appears to be that which is foremost in it,’ as the Philosopher states (Ethic., ix). That which is foremost in the Law of the novum testamentum, and in which all its power consists, is the grace of the Holy Spirit, which is given through faith in Christ. Consequently the lex nova is principally the grace itself of the Holy Spirit, which is given to those who believe in Christ.65
To establish his position he cites Jer 31:31,33, followed by two citations of Augustine from the De spiritu, the second of which reads: ‘What else are the Divine laws written by God Himself on our hearts, but the very presence of his Holy Spirit?’ From Augustine, Thomas reads the contrast in Jer 31 as between an old law without further power, and the ‘new’ work of the Spirit in those who believe.
The consequences of this for Jer 31 are then spelled out by an objection now somewhat familiar: what of those prior to the nova lex? If the new law is the Spirit’s work by which people are made friends of God, and if the ancient faithful had that Spirit’s work, then you have the novum testamentum in the era of the vetus. Thus, the objection runs, the new law cannot be defined this way:
The Law of the Gospel is proper to those who are in the state of the novum testamentum. But the Law that is inscribed [on the heart] is common both to those who are in the novum testamentum and those who are in the vetus testamentum. For it is said in Wisdom 7[:27]: ‘Divine wisdom conveys herself through the nations into holy souls; she establishes the friends of God and the prophets.’ Therefore the lex nova is not the Law inscribed.66
Thomas answers by appeal to an implicit distinction between membership in the novum testamentum and the ‘state (or era) of the novum testamentum’:
No one ever possessed the grace of the Holy Spirit except through faith in Christ, explicit or implicit. Through faith in Christ a man belongs to the novum testamentum. Thus whoever had the Law of grace infused, accordingly belonged to the novum testamentum
At first glance it does not appear that Thomas answers the objection. He solves the dilemma by agreeing that there have always been those who had the Law of grace and belonged to the novum testamentum. The implicit point, however, is that Thomas does not see ‘belonging to the novum testamentum’ as the same as being in the statu novi testamenti – otherwise the reply would not at all address the objection. Thomas thus drives a distinction between two realities, that of the era of the Gospel or the statu novi testamenti, and that of
membership within the novum testamentum. And Jer 31:33-34 is addressed to the latter. Thus, in article 4 of the same question he asserts that the state of the new law succeeds the state of the old law (‘successit enim status novae legis statui veteris legis’), a claim he finds consistent with the novum testamentum existing during the state of the old law.
Matthew Levering summarizes the distinction being made this way: ‘The state of the new law begins after the Incarnation, while the new law itself, as the grace of the Holy Spirit, is found in all places and times.’68 Or more fully is Colman O’Neill:
the new law exists as the mystery of salvation at work in the world from the time of the restoration of man to grace. Yet, though the new law thus transcends historical periods, the state of the new law does not. For the state of the new law is precisely that third state of revelation and faith which was initiated in the Incarnation and in the mysteries of Christ.69
That Thomas owes this position to Augustine is clear: Augustine is cited no fewer than 8 times in answer to this one question. One can speak two different ways of the vetus and novum testamentum (or lex): either to a ‘state’ or era, or with respect to the thing itself. If the former, then one can speak of the economical differences. But if the latter, then any view of temporal succession is impossible. And Jeremiah is speaking of the latter. The lex nova, spoken of in Jer 31:33-34, is available throughout all ages and without the possession of it, one’s happiness (proper end) is unattainable – for that which is outside of a person cannot justify. The virtue of being just before God cannot be acquired unless given by God, and clearly those faithful of the ancient era were just. Thus ‘in all times there have been some belonging to the novum testamentum’, even if the statu novi testamenti awaited the coming of Christ.70
This point is made concrete in Thomas’ treatment of David in Psalm 51 (Vg. 50) and Thomas’ view of David as having the Holy Spirit (i.e. the lex nova):
The reason for this manifestation [of guilt (culpa) being wiped clean] is a divine mercy; for the manifestation of righteousness (iustis) is useful so that we do not presume on his righteousness (iustitia). For if David sins – after all of his victories, after the gift of the Holy Spirit, after all his familiarity with God and prophecy – how much more ought we to fear how weak and sinful we are?71
If Jeremiah’s prophecy of the new covenant is a prophecy of the lex nova, which is contained fundamentally in the giving of the Holy Spirit, then David is here explicitly counted as a member of the new covenant. The exhortation even hinges upon an a fortiori privileging of the place of David: if even David can sin, how much more should we fear? There is only one way by which anyone is made right with God, and that is through the novum testamentum or the lex nova, which is the grace of the Holy Spirit given to those who believe. This is true for Augustine and Thomas regardless of era, and this right standing before God is the substance of Jeremiah’s new covenant.72