Reformed Christians speak of Scripture as the unfolding drama of God's covenant of grace. We do this because the apostle Paul speaks of the Israelites, saying, "To them belong … the covenants" (Rom. 9:5). The Bible is a covenantal story, and one that Paul, again, describes as "the covenants of promise" (Eph. 2:12).
The essence of the covenant of grace is the same throughout the Old and New Testaments—God saves sinners by grace alone, through faith alone, in Christ alone. But its historical administration has varied by time and place. For example, the covenant of grace widened from the Old Testament to the New Testament, as it was administered first with small families (e.g., the families of Noah and Abram), then with the nation of Israel, but now with the church, which is made up of people "from every tribe and language and people and nation" (Rev. 5:9). Also, it was administered in the Old Testament through what the New Testament authors describe as "types" and "shadows" (Heb. 8:5; 10:1), such as sacrifices, the priesthood, and the temple, all of which pointed to their reality, Jesus Christ (e.g., Col. 2:17).
The Reformed creeds and confessions express the continuity of God's covenant of grace despite its many historical variations. For instance, the Heidelberg Catechism says: "… God himself first revealed [it] in Paradise, [and] afterwards [it was] proclaimed by the holy Patriarchs and Prophets, and foreshadowed by the sacrifices and other ceremonies of the law, and finally fulfilled in his well-beloved Son" (Q&A 19). This means the Bible is one story of the gospel, which God has spoken "in many times and in many ways" (Heb. 1:1), whether in Paradise to Adam; during the days of the patriarchs, such as Noah, Abraham, Isaac, Jacob, and Moses; through the ministry of the prophets, such as Isaiah, Jeremiah, Hosea, and Joel; or through the ceremonies of the Levitical sacrifices. All of this came to fruition in Jesus Christ.
Likewise, while recognizing the variations in the administration of the covenant of grace between the Old and New Testaments, the Westminster Confession of Faith affirms the continuity of the covenant in the promise of Christ and His fulfillment of it:
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 fore-signifying 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.
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; which, though fewer in number, and administered with more simplicity and less outward glory, yet in them, it is held forth in more fullness, evidence, and spiritual efficacy, to all nations, both Jews and Gentiles; and is called the New Testament. There are not, therefore, two covenants of grace differing in substance, but one and the same under various dispensations. (7.5-6)
When our Lord Jesus Christ was born, lived, died, and was raised from the grave, the covenant of grace reached its zenith in what the Bible calls "the new covenant" (Jer. 31:31; Luke 22:20; 1 Cor. 11:25; 2 Cor. 3:6; Heb. 8:8, 13; 9:15; 12:24). Under the covenant of grace, Christ accomplished what Adam failed to do in the covenant of works, so we receive grace:
Man's work faileth, Christ's availeth;
He is all our righteousness;
He, our Savior, has forever
Set us free from dire distress.
Through His merit we inherit
Light and peace and happiness.