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.
Herman Bavinck, Reformed Dogmatics, 3:223
