Louis Berkhof says in his Systematic Theology that Law is a means of grace. Would the OPC agree with that?
Before I begin, let me note two things about the question: (1) that you refer specifically to Louis Berkhof's Systematic Theology and (2) that you refer specifically to his treatment of the "means of grace" (which Berkhof deals with in Part Five, "The Church and the Means of Grace," ecclesiology) rather than to his treatment of law and grace in general or, say, to his treatment of the covenant of grace (which Berkhof deals with—and quite properly so—in Part Two, "The Doctrine of Man in Relation to God," anthropology, where it properly belongs in a systematic theology).
Incidentally, I'll be quoting primarily from Louis Berkhof, Systematic Theology (Eerdmans, 1941), but occasionally from his Summary of Christian Doctrine (Eerdmans, 1938) for the sake of brevity.
In the Reformed tradition, to speak of the "means" of grace is to speak of something very specific, viz., the Word of God, the sacraments, and (at least for Presbyterians) prayer. We see this use, for example, in both the Westminster Larger and Shorter Catechisms:
WLC Q. 154. What are the outward means whereby Christ communicates to us the benefits of his mediation? A. The outward and ordinary means whereby Christ communicates to his church the benefits of his mediation, are all his ordinances; especially the word, sacraments, and prayer; all which are made effectual to the elect for their salvation.
WSC Q. 88. What are the outward and ordinary means whereby Christ communicateth to us the benefits of redemption? The outward and ordinary means whereby Christ communicateth to us the benefits of redemption are, his ordinances, especially the Word, sacraments, and prayer, all which are made effectual to the elect for salvation.
Likewise Louis Berkhof in his discussion of "the means of grace" in his Systematic Theology speaks not only of "The Word as a Means of Grace" (p. 849), but also of "The Sacraments" (p. 620), specifically "Christian Baptism" (p. 622) and "The Lord's Supper" ([p. 644).
Where does law fit into the picture? As Berkhof expounds it, the Word of God as a means of grace is comprised of law and gospel. Here is how he describes it on p. 612 of his Systematic Theology:
The churches of the Reformation from the very beginning distinguished between the law and gospel as the two parts of the Word of God as a means of grace. This distinction was not understood to be identified with that between the Old and the New Testament, but was regarded as a distinction that applies to both Testaments. There is law and gospel in the Old Testament, and there is law and gospel in the New.
He treats this subject at some length, but it is too detailed to quote it at length here. Rather, we'll let Berkhof summarize it himself in his Summary of Christian Doctrine, pp. 164-165:
"The Word of God is the most important means of grace....The Word as a means of grace consists of two parts, namely, the law and the gospel. The law as a means of grace first of all serves the purpose of bringing men under conviction of sin, Romans 3:20, making him conscious of his inability to meet the demands of the law, and becoming his tutor to lead him to Christ, Gal. 3:24. In the second place it is also the rule of life for believers, reminding them of their duties and leading them in the way of life and salvation. The gospel is a clear representation of the way of salvation revealed in Jesus Christ. It exhorts the sinner to come to Christ in faith and repentance, and promises those who truly repent and believe all the blessings of salvation in the present and in the future. It is the power of God unto salvation for every one that believeth. Rom. 1:16; 1 Cor. 1:18.
So the short answer to your question is, yes. The OPC would agree with that (because it agrees with our Standards). Inasmuch as the Law is God's Word, then it is a means of Grace—when it is read, but especially when it is preached. So, when and where the Law is being faithfully read and preached, the Holy Spirit uses it as a means by which God's elect are "enlightened, convinced, and humbled" and "built up in grace, holiness, and comfort," etc.:
WLC Q.155. How is the word made effectual to salvation? A. The Spirit of God maketh the reading, but especially the preaching of the word an effectual means of enlightening, convincing, and humbling sinners; of driving them out of themselves, and drawing them unto Christ; of conforming them to his image, and subduing them to his will; of strengthening them against temptations and corruptions; of building them up in grace, and establishing their hearts in holiness and comfort through faith unto salvation.
WSC Q. 89. How is the Word made effectual to salvation? The Spirit of God maketh the reading, but especially the preaching of the Word, an effectual means of convincing and converting sinners, and of building them up in holiness and comfort, through faith, unto salvation."
We see here (although the Westminster Larger and Shorter do not make use of this terminology) two of the three uses in the threefold use of the law, a point already made in the previous quotation from Berkhof's Summary of Christian Doctrine:
The law as a means of grace first of all serves the purpose of bringing men under conviction of sin, Romans 3:20, making him conscious of his inability to meet the demands of the law, and becoming his tutor to lead him to Christ, Gal. 3:24. In the second place it is also the rule of life for believers, reminding them of their duties and leading them in the way of life and salvation.
Here we have Berkhof's specific teaching on "the law as a means of grace," and we see both the second use and the third use at work.
Here is how Berkhof describes"the threefold use of the law" in his Systematic Theology (pp. 614-615):
a. A usus politicus or civilis. The law serves the purpose of restraining sin and promoting righteousness.... It serves the purpose of God's common grace in the world at large. This means that from this point of view it cannot be regarded a means of grace in the technical sense of the word.
b. A usus elenchticus or pedagogicus. In this capacity the law serves the purpose of bringing man under conviction of sin, and of making him conscious of his inability to meet the demands of the law. In that way, the law becomes his tutor to lead him unto Christ, and thus becomes subservient to God's gracious purpose of redemption.
c. A usus didacticus or normativus. This is the so-called tertius usus legis, the third use of the law. The law is a rule of life for believers, reminding them of their duties and leading them in the way of life and salvation.
It's interesting that John Calvin regarded this third use as the primary use of the law. Presbyterians (including the OPC) have followed Calvin in recognizing the importance of the third use of the law.
Finally, as we consider the Word of God (incorporating law and gospel) as a means of grace, we note that the Westminster Confession of Faith, Chapter 19, "Of the Law of God"—although it never uses the phrase "means of grace"—yet is in full agreement with what Louis Berkhof expounds in that area. Here is the Westminster Confession of Faith, Chapter 19, Section 6:
Although true believers be not under the law, as a covenant of works, to be thereby justified, or condemned; yet is it of great use to them, as well as to others; in that, as a rule of life informing them of the will of God, and their duty, it directs and binds them to walk accordingly; discovering also the sinful pollutions of their nature, hearts, and lives; so as, examining themselves thereby, they may come to further conviction of, humiliation for, and hatred against sin, together with a clearer sight of the need they have of Christ, and the perfection of his obedience. It is likewise of use to the regenerate, to restrain their corruptions, in that it forbids sin: and the threatenings of it serve to show what even their sins deserve; and what afflictions, in this life, they may expect for them, although freed from the curse thereof threatened in the law. The promises of it, in like manner, show them God's approbation of obedience, and what blessings they may expect upon the performance thereof: although not as due to them by the law as a covenant of works. So as, a man's doing good, and refraining from evil, because the law encourageth to the one, and deterreth from the other, is no evidence of his being under the law; and, not under grace.
There is one place where Presbyterians (including the OPC) would differ with Louis Berkhof, but it is not with his exposition of the Word of God (including law and gospel) as a means of grace. Berkhof restricts the means of grace to the Word of God and the sacraments. In keeping with the Westminster Larger and Shorter Catechisms, Presbyterians historically (including Charles Hodge in his own Systematic Theology) have considered prayer to be also a means of grace.
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