Overemphasizing the disunity of Scripture can be problematic and it has proved so throughout Church history. We think of groups ranging from Marcion and the Gnostics to some of the Anabaptists and most certainly modern day Dispensationalism which dominates the Evangelical scene. Ironically over the past twenty years a new hybrid has been created, Dispensationalism rooted in disunity wedded to Dominionism which over-emphasizes unity. And yet the resulting theology is not a happy medium or evenly balanced. It represents a double-distortion and two-fold departure from Scriptural frameworks and doctrine. It is neither consistent, nor coherent. It is not Scriptural.
This question, the relation of the Old and New Testaments or Covenants is in fact one of the great issues in all of Church history. How you answer this question affects a host of issues and in fact your very understanding of what the Church is and what it is here to do.
Throughout most of Church history the main problem has been the tendency to overemphasize the 'unity' between the covenants. Too often and without principled thought or purpose people will jump to the Old Testament to justify everything from worship practices to nationalism and war.
Something profound changed when the veil of the temple was rent. The old order of the Temple, the priesthood, a holy land and holy wars all came to end or would in about forty years... a significant number in itself. By the year 70, the Temple order had terminated. Contrary to the assertions of modern day Dispensationalism, this was the fulfillment of Daniel 9. That passage does not hint of something awaiting realization during a future seven year tribulation.
And yet for all the emphasis on disunity, the New Testament also teaches an encompassing and organic unity. Christ is the center of the story from beginning to end. Israel itself is but another name, a picture, a type and symbol of Jesus Christ.
In Ephesians the Church is identified as part of the Commonwealth of Israel. The story of Israel does not end when the old order passes away. The definition of Israel is redefined and expanded upon. The Jews are dispossessed of the Kingdom (Matt 21.43) but can still enter the new Kingdom by the same faith as Gentile believers. In fact that's how it was all along Paul teaches us in Galatians 3. Only those who are of faith are the real sons of Abraham, the real Israelites as it were. This is emphasized again in passages such as Ephesians 2 and Romans 9-11.
This question of Unity and Disunity must be wrestled with. It affects our understanding of these wedded doctrines... the Kingdom, Israel, the Church and the Covenant. Without the New Testament we cannot properly relate these nuances of a common idea. In the case of the covenants the passage in Ephesians binds their plurality with the one promise that is Christ.
There is an overarching story, God declaring to us He will be our God and we will be His people. The heart of the story is Christ. All the promises are yes and amen, affirmed and confirmed in Jesus Christ (2 Cor 1.20). All other fulfillments are temporary and typological. Today we must read the whole of the Old Testament in light of Jesus Christ the final prophet as He is identified in the opening chapter of Hebrews.
Ephesians 2.20 tells us the New Covenant Temple, the Israel of God, is built on the foundation of the apostles and prophets. It is the testimony of the apostles bearing Christ's promise of the Holy Spirit that establishes for us the New Covenant Temple, the Church, the Body of Christ, the Kingdom of God but also teaches us how to rightly read, understand and interpret the Old Testament. We cannot rightly know and appreciate the Old without the information given to us in the New.
Without the New Testament's interpretation of the Old we are likely to fall into the same Judaizing trap which ensnared the Pharisees. They looked for a Messiah that would be another Alexander, who would destroy Rome and establish a Jewish Empire. Many would say they simply misunderstood the timetable and that will indeed come to pass in the future. Instead Christ and the Apostles taught they misunderstood not just the timetable but the whole nature of the Kingdom itself. They had misunderstood the message of the Old Testament and thus when the Messiah appeared they hated him and wished him dead.
Many theologies of our day from Roman Catholicism to a great deal of Protestant theology also falls into a similar morass and ends up reading the New in light of the Old. This is an inversion and Paul refers to it as a veil in 2 Corinthians 3. The Jews refusing to read the Old Testament in light of the revelation of Christ remained in bondage and blind to the Kingdom. It reminds of Christ's words to Nicodemus in John 3. You must be born again to 'see' the Kingdom of God. It's a Kingdom that doesn't come with observation, a Kingdom of righteousness, peace and joy in the Holy Spirit. It's a Kingdom not of this world (John 18.36).
We do not ignore the Old Testament but we recognize the supremacy of the New Testament. It's a better covenant (Heb 8.6) established on better promises that are permanent and eternal.
Bringing the Old Covenant mindset to the New Testament leads to a distortion of the Church and its martyr-witness in this age. Theology is deformed and when people and nations act as Old Covenant Israel they are not the agents of God bringing righteous judgment, instead they are men setting themselves up as God and with evil motives that are guilty of doing evil deeds. They are also in danger of looking for prophetic fulfillment 'outside' of Christ. If they think the Old Testament prophecies are yet to be fulfilled in the context of land and temples, then they have gravely misunderstood the Christocentric (Christ-centered) nature of the New Testament and how it understands Old Testament prophecy.
We are not New Testament only Christians but the New Testament is officially our canon. The Old Testament while necessary and relevant is now obsolete. Hebrews 7.12-18 makes this clear. That does not render it worthless but its value is determined and understood in light of the new revelation, the new covenant that has superseded it. We cannot understand the New without the context of the Old but the Old cannot be rightly understood unless it is read in light of the revealed Christ. Christ and the apostles elucidate the Old and help us to understand its message and how to rightly read it and apply it. The New establishes our hermeneutics for reading the Old. And because of the unity there are ideas and concepts present in the Church that many end up missing.
In this sense we can say we are New Testament or New Covenant Christians. In another sense every believer in history, every person who has been saved was a member of the New Covenant... even if they lived in the time before it was fully revealed.
In that sense every Christian is a New Testament Christian. The New Testament is supreme.