The Old and the New Testaments are in essence one covenant (Luke 1:68–79; Acts 2:39; 3:25). They have one gospel (Rom. 1:2; Gal. 3:8; Heb. 4:2, 6; 2 Tim. 3:15); one mediator, namely, Christ, who existed also in the days of the Old Testament (John 1:1, 14; 8:58; Rom. 8:3; 2 Cor. 8:9; Gal. 4:4; Phil. 2:6; etc.), exercised his office of mediator (John 8:56; 1 Cor. 10:4; 1 Pet. 1:11; 3:19; Heb. 13:8), and is the only mediator for all humans and in all times (John 14:6; Acts 4:12; 1 Tim. 2:5). It included one faith as the way of salvation (Matt. 13:17; Acts 10:43; 15:11; Rom. 4:11; Gal. 3:6–7; Heb. 11); the same promises and benefits of God’s communion (2 Cor. 6:16; Rev. 21:3), forgiveness, justification (Acts 10:43; Rom. 4:22), and eternal life (Matt. 22:32; Gal. 3:18; Heb. 9:15; 11:10; etc.). The road was the same on which believers in the Old and the New Testaments walked, but the light in which they walked was different.
For all the unity between them, therefore, there is also difference. The Old and the New Testaments as different dispensations of the same covenant of grace are related as promise and fulfillment (Acts 13:32; Rom. 1:2), as shadow and substance (Col. 2:17), as the letter that kills and the Spirit that makes alive (2 Cor. 3:6ff.), as servitude and freedom (Rom. 8:15; Gal. 4:1ff., 22ff.; Col. 2:20; Heb. 12:18f.), as particular and universal (John 4:21; Acts 10:35; 14:16; Gal. 4:4–5; 6:15; Eph. 2:14; 3:6).

The new thing in the New Testament, therefore, is the shedding of the nonarbitrary but still temporary sensory national forms under which one and the same grace was revealed in the old day. The new dispensation already starts in a sense when, with the births of John the Baptist and Jesus, the promises of the Old Testament begin to be fulfilled. Yet the old dispensation still remained in effect up until the death of Christ. Jesus himself was an Israelite, fulfilled all righteousness, and still concerned himself only with the lost sheep of the house of Israel. But at his death, the curtain in the temple tore in two (Matt. 27:51), the testator himself died (Heb. 9:15–17), the New Testament was founded in his blood (Matt. 26:28), the bond that stood against us was canceled (Col. 2:14), the dividing wall was broken down (Eph. 2:14), and so on. Factually the old dispensation may long linger, but legally it has been abolished. Better still, nothing was abolished, but the fruit was ripe and broke through the husk. The church, carried like a fetus in Israel’s womb, was born to an independent life of its own and in the Holy Spirit received an immanent life principle of its own. The sun of righteousness rose to its zenith in the heavens and shone out over all peoples. The law and the prophets have been fulfilled and in Christ as their end and goal reached their destiny. The law was given through Moses; grace and truth came through Jesus Christ (John 1:14). He is the truth (John 14:6), the substance (Col. 2:17) in whom all the promises and shadows have been realized. In him all things have been fulfilled. He is the true prophet, priest, and king; the true servant of the Lord, the true expiation (Rom. 3:25), the true sacrifice (Eph. 5:2), the true circumcision (Col. 2:11), the true Passover (1 Cor. 5:7), and therefore his church is the true seed of Abraham, the true Israel, the true people of God (Matt. 1:21; Luke 1:17; Rom. 9:25–26; 2 Cor. 6:16–18; Gal. 3:29; Titus 2:14; Heb. 8:8–10; 1 Pet. 2:9; Rev. 21:3), the true temple of God (1 Cor. 3:16; 2 Cor. 6:16; Eph. 2:21; 2 Thess. 2:4; Heb. 8:2, 5), the true Zion and Jerusalem (Gal. 4:26; Heb. 12:22), its spiritual offering, the true religion (John 4:24; Rom. 12:1; Phil. 3:3; 4:18; etc.). Nothing of the Old Testament is lost in the New, but everything is fulfilled, matured, has reached its full growth, and now, out of the temporary husk, produces the eternal core. It is not the case that in Israel there was a true temple and sacrifice and priesthood and so on and that all these have now vanished. The converse, rather, is true: of all this Israel only possessed a shadow, but now the substance itself has emerged. The things we see are temporal, but the invisible things are eternal.

Herman Bavinck, ed. John Bolt and John Vriend, Reformed Dogmatics: Sin and Salvation in Christ, vol. 3 (Grand Rapids, MI: Baker Academic, 2006), 223–24 (HT: Chad Vegas).