John Fesko on the Westminster Standards...

"The divines never formally addressed the matter of subscription (the manner and degree to which ministers and elders were required to adhere to the Standards), but at two points--the republication of the covenant of works and the covenant of redemption---a principled diversity of views existed behind the scenes. This plurality of views confirms that the divines never intended the Confession to be a doctrinal straightjacket but instead a corporate confession for the church, not the manifesto of one particular party." 

(John Fesko, The Theology of the Westminster Standards, p 167)

The Covenant of Grace - In the time of the Law... In the time of the Gospel (3)

The question continues in this form: How was the covenant of grace administered in the Old Testament or in the time of the law and how is it administered in the New Testament or under the gospel? Continuity?

In his book, The Westminster Assembly, Robert Letham makes an assertion on page 233 that many today may assume to be correct,...

 both the Confession and the Larger Catechism say that law and gospel are  different means of administering the one covenant of grace... 

And on page 234 he writes: 

WCF 7.5 spells out clearly that the law was an administration of the covenant of grace.

One may assert that position, but the WCF and WLC do not state that conclusion:

WCF 7.5. This covenant was differently administered in the time of the law, and in the time of the gospel: under the law, it was administered by promises, prophecies, sacrifices, circumcision, the paschal lamb, and other types and ordinances delivered to the people of the Jews, all foresignifying Christ to come; which were, for that time, sufficient and efficacious, through the operation of the Spirit, to instruct and build up the elect in faith in the promised Messiah, by whom they had full remission of sins, and eternal salvation; and is called the old testament.

WLC Q. 34. How was the covenant of grace administered under the Old Testament?

A. The covenant of grace was administered under the Old Testament, by promises, prophecies, sacrifices, circumcision, the passover, and other types and ordinances, which did all foresignify Christ then to come, and were for that time sufficient to build up the elect in faith in the promised messiah, by whom they then had full remission of sin, and eternal salvation.

The Westminster Divines did not teach that the law was a means of administering the covenant of grace. Chad Van Dixhoorn writes in his book, Confessing The Faith:

... we are reminded that the covenant of grace was 'administered' differently 'in the time of the law, and in the time of the gospel'. The one time is symbolized by the law engraved on stones... p 103

Yet!...

The redeeming feature of this time of the law was that these types and ordinances all pointed directly to a coming Christ... p 104

Nowhere in the Westminster Standards does it state that the covenant of grace in the Old Testament was administered by the law or by the commandments. This is not to say that the law commandments weren't and aren't essential to the outworking of the covenant of grace in God's work of bringing sinners to Christ and sanctifying the elect. Yet properly, the law itself speaks only of our duty before God and its condemnation of sinners for the breach of it. As its primary use (WLC 95, 96, 97) the law, through the work of the Holy Spirit, shines a light on our corruptions and lack of conformity to it. The law points sinners to Christ. Sinners, both unregenerate and regenerate, convicted by the law through the agency of the Holy Spirit are driven to see their need of the Savior and further, in the lives of the redeemed, increases both their gratitude to Christ and their desire to conform to its commands. 

WLC Q. 97. What special use is there of the moral law to the regenerate? 

A. Although they that are regenerate, and believe in Christ, be delivered from the moral law as a covenant of works, so as thereby they are neither justified nor condemned; yet besides the general uses thereof common to them with all men, it is of special use, to show them how much they are bound to Christ for his fulfilling it, and enduring the curse thereof in their stead, and for their good; and thereby to provoke them to more thankfulness, and to express the same in their greater care to conform themselves thereunto as the rule of their obedience.

But in the law no grace or power is offered to sinners for cleansing from or the overcoming of sin. As John Owen wrote, 

The law guides, directs, commands, all things that are against the interest and rule of sin. It judgeth and condemneth both the things that promote it [sin] and the persons that do them; it frightens and terrifies the consciences of those who are under its [sin's] dominion. But if you shall say unto it [the law], “What then shall we do? this tyrant, this enemy, is too hard for us. What aid and assistance against it [sin] will you afford unto us? what power will you communicate unto its destruction?” Here the law is utterly silent, or says that nothing of this nature is committed unto it of God: nay, the strength it hath it gives unto sin for the condemnation of the sinner: “The strength of sin is the law.” (A Treatise of the Dominion of Sin and Grace)

What then, or Who, will then come to the aid of sinners under this heavy load? John Owen continues...

But the gospel, or the grace of it, is the means and instrument of God for the communication of internal spiritual strength unto believers. By it do they receive supplies of the Spirit or aids of grace for the subduing of sin and the destruction of its dominion…

Indeed, WCF 7.6 teaches that in the New Testament we find the covenant of grace fulfillment of not the law but the Old Testament types and shadows... 

Under the gospel, when Christ, the substance, was exhibited, the ordinances in which this covenant is dispensed are the preaching of the Word, and the administration of the sacraments of baptism and the Lord's Supper... 

In this time of the gospel, it is now these three New Testament ordinances by which God administers the covenant of grace to his people. These ordinances are not law, but the verbal and visible proclamations of the grace of God offered to sinners - Christ offered in the gospel -- the fulfillment of the promises, types, and shadows of the Messiah found under the law and in the time of the law. Christ was 'the substance' of the covenant of grace then promised but now revealed in the time of the gospel. God's means of administering the covenant of grace, under or in the law with the Old Testament types and shadows, did not change in the New Testament, but rather found its fulfillment in Christ under the gospel.

Q. 35. How is the covenant of grace administered under the New Testament? 

A. Under the New Testament, when Christ the substance was exhibited, the same covenant of grace was and still is to be administered in the preaching of the word, and the administration of the sacraments of baptism and the Lord's supper; in which grace and salvation are held forth in more fullness, evidence, and efficacy, to all nations.

Herman Witsius: "The covenant made with Israel at Sinai was..." (2)

The Question under consideration is how was the Covenant of Grace administered "in the time of the law?"

Herman Witsius, The Economy of the Covenants:

"Having premised these observations I answer to the question: The covenant made with Israel at mount Sinai was not formally the covenant of works...

"Nor was it formally a covenant of grace: because that requires not only obedience, but also promises, and bestows strength to obey. For thus the covenant of grace is made known, Jer. xxxii.39. "And l will give them one heart, and one way, that they may fear me forever." But such a promise appears not in the covenant made at mount Sinai. Nay, God, on this very account, distinguishes the new covenant of grace from the Sinaitic, Jer. xxxi. 31, 32, 33. And Moses loudly proclaims, Deut. xxix. 4. "Yet the Lord hath not given you a heart to perceive, and eyes to see, and ears to hear, unto this day." Certainly, the chosen from among Israel had obtained this: yet not in virtue of this covenant, which stipulated obedience, but gave not power for it; but in virtue of the covenant of grace, which also belonged to them...

"What was it then? It was a national covenant between God and Israel, whereby Israel promised to God a sincere obedience to all his precepts, especially to the ten words; God, on the other hand, promised to Israel, that such an observance would be acceptable to him, nor want its reward, both in this life, and in that which is to come, both as to soul and body. This reciprocal promise supposed a covenant of grace. For, without the assistance of the covenant of grace, man cannot sincerely promise that observance; and yet that an imperfect observance should be acceptable to God, is wholly owing to the covenant of grace. It also supposed the doctrine of the covenant of works, the terror of which being increased by those tremendous signs that attended it, they ought to have been excited to embrace that covenant of God. This agreement therefore is a consequent both of the covenant‘ of grace and of works; but was formally neither the one nor the other." pp 34, 36

"In the time of the law..." (1)

What did the Westminster Divines mean by "in the time of the law" and in what way was the Covenant of Grace administered during that period of redemptive history...

Westminster Confession of Faith. Chapter 7:

4. This covenant of grace is frequently set forth in Scripture by the name of a testament, in reference to the death of Jesus Christ the Testator, and to the everlasting inheritance, with all things belonging to it, therein bequeathed. 

5. This covenant was differently administered in the time of the law, and in the time of the gospel: under the law, it was administered by promises, prophecies, sacrifices, circumcision, the paschal lamb, and other types and ordinances delivered to the people of the Jews, all foresignifying Christ to come; which were, for that time, sufficient and efficacious, through the operation of the Spirit, to instruct and build up the elect in faith in the promised Messiah, by whom they had full remission of sins, and eternal salvation; and is called the old testament. [footnote in original Gal.3:7-9,14]

A.A. Hodge wrote concerning the above: 

"Under the old dispensation the covenant of grace was administered chiefly by types and symbolic ordinances, signifying beforehand the coming of Christ, and thus administration was almost exclusively confined to the Jewish nation with constantly increasing fullness and clearness- (1) From Adam to Abraham, in the promise to the woman (Gen. 3:15); the institution of bloody sacrifices; and the constant visible appearance and audible converse of Jehovah with his people. (2) From Abraham to Moses, the more definite promise given to Abraham (Gen. 17:7; 22:18), in the Church separated from the world, embraced in a special covenant, and sealed with the sacrament of Circumcision. (3) From Moses to Christ, the simple primitive rite of sacrifice developed into the elaborate ceremonial and significant symbolism of the temple service, the covenant enriched with new promises, the Church separated from the world by new barriers, and sealed with the additional sacrament of the Passover."Hodge, A.A., A Commentary on The Westminster Confession of Faith

Is the Mosaic Covenant, strictly speaking, best described as The covenant of grace or better as an administration of the covenant of grace? And is there a difference? Some say no! Some say yes. Part of the problem in answering that question is that, as I and others have pointed out, the term Mosaic Covenant is not a Biblical term nor a confessional term. There was a specific covenant given at Sinai through Moses that Scripture often refers to as the Law (Galatians 3:17). But within the continuing dispensation of the time of the law were different elements which served different functions. In a word, there were both Law and Gospel in the Mosaic economy. Being that they are not the same (Galatian 3:12), they had very different purposes or functions. Yet even those conditional legal elements served to further the unfolding Covenant of Grace in history. And not all elements of the Mosaic economy (the time of the Law from Moses to Christ) were, by any means, included in the Sinai covenant given at Mt. Horeb. Some were. Some were not.

There is plenty of precedent for understanding the Mosaic Economy/Covenant as a mixed covenant (e.g. Hodge, both Charles and A.A.) and also one not strictly or solely of grace or works (see Witsius next post). Of interest to me is that WCF 7.5 does not say the Law was, or even administered, the Covenant of Grace, but rather that "in the time of the law" and "under the law" the Covenant of Grace was administered... by what? By "promises, prophecies, sacrifices, circumcision, the paschal lamb, and other types and ordinances delivered to the people of the Jews, all foresignifying Christ to come." Conspicuously absent from this list of administration elements are any conditional legal works references which are nonetheless very prominent during the Mosaic covenant/economy inaugurated at Sinai (e.g. Deuteronomy 28 among many references).