During the second century, the heretic Marcion questioned the church's received canon of Scripture. Marcion belonged to the Gnostic movement, which among other things taught that the God revealed in the Old Testament, Yahweh, is different from the God revealed in the New Testament, the Father of our Lord Jesus Christ. In so doing, Marcion and the other Gnostics set the new covenant Scriptures against the old covenant Scriptures such that the New Testament contradicted the Old Testament.
Marcion denied the longstanding Christian belief that although there are differences between the Old and New Testaments, such differences do not amount to contradictions. In fact, Jesus and the Apostles quote and allude to the Old Testament so often that one must excise, as Marcion did, all of the Old Testament references from the New Testament in order to believe that each testament reveals a different deity. From Jesus' statement that He came not to abolish the Law or the Prophets to the author of Hebrews' statement that the God who spoke through the old covenant prophets is the same God who has spoken through the Son, it is clear that the Old and New Testaments both reveal the same Creator (Matt. 5:17; Heb. 1:1–2).
Yet Christians have long affirmed that this Creator did not reveal Himself to human beings all at once, and this helps to account for many of the differences between the old covenant and the new covenant. Scripture teaches what is often called "progressive revelation." Simply put, this means that God revealed Himself to His people over many centuries, periodically giving new information that built on but did not contradict or deny what came before. For example, the Lord spoke to Abraham and gave him the promise of salvation. Later, He spoke to Israel through Moses, the old covenant mediator, adding the law, which did not overturn the promise but rather reinforced Israel's hope in the promise (Gal. 3:15–29). After that, the prophets gave more revelation regarding God, and then the Lord's final revelation came in the incarnation of His Son and the Apostolic writings that explain His person and work (Heb. 1:1–2).
Progressive revelation means that while Scripture's application to old covenant believers is different in some ways from its application to new covenant believers, we do not reject any of God's Word. It reveals one message of salvation that we cannot fully grasp unless we consider the whole of the Bible.
Progressive revelation means that we who live in the new covenant era are blessed to know more about God's plan and its outworking than those who lived under the old covenant. That does not mean, however, that we ignore the old covenant revelation, for Scripture cannot be broken (John 10:35). We need the Old Testament as much as we need the New Testament to know our Creator and how to love and obey Him.