While Christianity and Judaism share the foundation of the Old Testament scriptures, the central difference is the identity of Jesus of Nazareth: Christians believe He is the promised Messiah and the fulfillment of all Old Testament types and prophecies, while Judaism holds that the Messiah has not yet come. This creates distinct views on salvation, the efficacy of the law, and the necessity of Christ's substitutionary atonement for the forgiveness of sins.
The Reformed tradition, and Particular Baptists in particular, maintain that the Old and New Testaments are unified in their substance, as both present Christ as the only Mediator (39 Articles of Religion Art. VII). Christians believe that the Old Testament ceremonies and types—such as the priesthood, the temple, and animal sacrifices—were shadow-preparations for the reality of Christ, who has now come. Consequently, as On the Incarnation of the Word Sec. 40 observes, with the arrival of the 'Holy of Holies,' the old system of prophecy and animal sacrifice in Jerusalem reached its divinely intended end. The differences between the Testaments are matters of administration rather than substance (Institutes of the Christian Religion Ch. 11 §1).
A critical theological divergence concerns how a person stands righteous before God. Historic Reformed theology, including the 1689 LBCF Ch. 11 §6, asserts that the justification of believers under the Old Testament was the same as under the New: by grace through faith in the promised Messiah. Judaism today typically emphasizes covenantal living and the observance of the Law (mitzvot). In contrast, the Christian faith, as articulated in the Second Helvetic Confession Ch. XV, teaches that we are justified by faith alone, apart from the works of the law, solely because Christ bore our sins and imputed His righteousness to us.
Ultimately, the gospel message is that we are more sinful than we dared believe, yet more loved in Christ than we dared hope. Christianity claims that Jesus is the exclusive way to the Father, a claim that distinguishes it from the broader scope of contemporary Judaism. As it is written: "Jesus saith unto him, I am the way, and the truth, and the life: no one cometh unto the Father, but by me" (John 14:6). This is the assurance that our standing before God rests entirely upon the finished work of Christ rather than our own efforts: "And in none other is there salvation: for neither is there any other name under heaven, that is given among men, wherein we must be saved" (Acts 4:12).