I've seen different pastors use the Old Testament to fill gaps and sometimes to override information in the New Testament. However, the New Testament is supposed to be part of the new deal which is to describe the new covenant post-Jesus.
What purpose does the Old Testament have in Christanity today? Should it still be part of the Bible?
Well, the Apostle Paul specifically states that the things written in the past were for our instruction, for encouragement, and to give us hope.
For whatever was written in former days was written for our instruction, that through endurance and through the encouragement of the Scriptures we might have hope. Romans 15:14 ESV
Also, the Old Testament is not merely a bunch of rules. On the contrary, it is filled with stories of God's faithfulness, God's ways, God's purposes, God's power, God's mercy, God's justice, God's plan, and God's heart.
Indeed, even though the ceremonial laws of the temple do not apply directly to us today, the story of the Creation, the Exodus, the Fall, God's grace to Hannah and Rebekah, and many other things all reveal God to us.
We can have confidence that as God has delivered His people in the past, He can do so today. Since God has answered prayers in the past, He will answer prayers today. Since God saw the affliction of His people in Egypt, He will see the affliction of His people today. Since God executed justice in the past, He will certainly do so today.
So, no, Levitical laws do not apply to us today. We are, indeed, under the New Covenant. Yet the Old Testament still reveals God to us.
Additionally, it also gives us the promise of the Messiah and the prophecies that were later fulfilled in Jesus, giving us confidence to its truthfulness.
The New Covenant allows us to know God more fully. The Old Testament help to teach us who He is.
Without the Old Testament, the New Testament has no foundation. It would make no sense.
The key doctrine of Christianity is that Jesus came to save us from sin, and we need such a savior because we are all born sinners. But how did we come to all be born sinners? Because of Adam. See 1 Cor 15:20-28, which explicitly discusses Adam, who of course is from the OT.
Another important doctrine is that Jesus was the perfect sacrifice, and thus satisfied the requirements of the sacrificial law. What sacrifical law is that? The laws of Moses, in the OT.
Without the Old Testament, the New Testament would be like saying, "The answer is 42", without telling you the question. Without knowing the question, the answer tells you nothing.
And by the way, Jesus specifically said that his coming did not make the Old Testament irrelevant or obsolete. Matt 5:"17 “Do not think that I came to destroy the Law or the Prophets. I did not come to destroy but to fulfill. 18 For assuredly, I say to you, till heaven and earth pass away, one jot or one tittle will by no means pass from the law till all is fulfilled. 19 Whoever therefore breaks one of the least of these commandments, and teaches men so, shall be called least in the kingdom of heaven; but whoever does and teaches them, he shall be called great in the kingdom of heaven."
Of course pretty much every Christian believes the Old Testament should still be part of our Bible as the shadows of Christ, still edify and prove that Jesus was the Christ:
All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness (2 Tomothy 3:16)
The New Testament is possibly not even understandable, without the gradual revelation of the Old Testament leading up to it. However, contrary to what people might think the meaning of the Old Testament 'as it should be applied today' is one of the most controversial subjects that can ever be raised among protestants. To explain the tension over the issue it is best to divide the Law into its three traditional parts and then comment on how Christians view them.
The law is commonly divided into ceremonial, moral and civil.
In Christianity it is agreed by most that Christ fulfilled the meaning of all that was ceremonial and that therefore when the curtain of the temple was torn from top to bottom these laws became 'obsolete'. From this standpoint all ceremonial laws, of which the High Priest offering sacrifices in the temple were probably the most sacred, are no longer application by obsolescence.
In the same manner most Christians believe that the civil laws that were used to govern Israel, such as the many capital punishments by stoning, burning, or strangulation ceased. These are also viewed as being symbolic of man's need for salvation under the 'curse of the Law'. Also as the invisible church of Christ replaced the literal outward church of the Jews, God no longer directly governs the Jewish people. There are some Christians who still think that God in some way governs the Jews. (I think dispensationalists still hold some ideas about this but am not sure of the details, as I am not that familiar with this unique view).
The controversy is mostly about how God’s moral law in the Old Testament should be applied today, or if there is any change to it at all. Most Christians who propose the idea that 'nothing changed' to God’s moral Law at all in Christ’s death, commonly refer to this verse where Jesus said:
"Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. (NIV Mathew 5:17)
A common verse referred to by those that believe the entire Law (moral, ceremonial, civil) was 'nailed to the cross', so that even its moral nature was modified in Christ, commonly refer to the verse:
For the law was given through Moses; grace and truth came through Jesus Christ. (NIV John 1:17).
Even among traditional reformed theologians many are divided over this question about the moral law. I am of the camp that you can’t divide the Law at all, so that all aspects were fulfilled in Christ and so nothing remains exactly as it was. For example, the Law said we should love our neighbor as ourselves, however it did not provide any ability to do so but condemned to hell any offence. In Christ however, under the liberty of grace, a stony heart has been replaced by a soft one, and an uncircumcised heart has been circumcised, therefore, Christians have love in the heart and can follow the command. Therefore, even the moral law has not gone un-crucified as it was formerly written on stone but now in a 'Spirit of liberty' it is written in a believer’s heart. In this way, Christ abolished even the moral law in the sense that he fulfilled all its requirements and then imputed that righteousness to believers, which in turn, infused the holiness required by the law as a basic 'principle of life and liberty' into Christians. In this sense the Bible say’s we are ‘no longer under law' (that is the moral law).
For sin shall not be your master, because you are not under law, but under grace. (Romans 6:14).
This last verse I quote is at the heart of the debate for depending on which theologian you consult the moral law is intended, or not intended, by this and similar verses. As for me I am absolutely convinced the moral law is chiefly intended as described above.
For details of specific theologians and their view this post explaining the difference between Biblical Theology and Covenant Theology. Biblical theology has the sole ambition of answering how the NT naturally develops from the OT. Covenant Theology carries both options about the view of the moral law. Another recent trend that has its own twist on the subject is called New Covenant Theology. I recommend reading Jown Owen’s and Luther’s view at the post, regarding how to best view the 'moral law'.
The Old Testament points to the New Testament. It bridges the gap between Creation and the Birth of Jesus for history, and from the Fall to the Resurrection for spiritual authority. As such, the Old Testament can inform and strongly influence how we interpret New Testament scripture. It has a place in establishing context for New Testament teachings. It is also the fore-runner to the New Testament, and therefore it can speak where the New Testament is silent. But it is now subordinate. It should not over-ride New Testament commands or teachings.
