The Old Testament was the law of God for the Israelites. In Deuteronomy 4:44 and 5:1, Moses makes it clear that the law was for the 12 tribes of Israel. It was not for any other people, except that some laws applied to those who would live with the Israelites (Numbers 15:29).
The Old Testament was never the law for Gentiles and today it is not even God’s law for the Jews. We must not try to take the laws in the Old Testament as the laws that God wants us to follow today. In Colossians 2:13-17, Paul makes it clear the Old Testament has been nailed to the cross. Because of that, no one is bound to keep the feast days, the Sabbath day, or anything else in the Old Testament. We must not try to make it our law today. Discussing how the priesthood changed from Levi to Judah when Christ became our high priest, the Hebrew writer states,
For when there is a change in the priesthood, there is necessarily a change in the law as well (Hebrews 7:12). The book of Hebrews is written to encourage the Jews to keep following the law of Christ and not go back to the Old Law which had finished its usefulness as a law.
Knowing these things, the question always comes up, “Why should I study the Old Testament?” If it is not the law for me today, what use is there in knowing what it says? Well, the Old Testament is still useful to us. In Romans 15:4, Paul says,
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. So, we still learn from the Old Testament! We get encouragement and hope from reading it. In 1 Corinthians 10:11, Now these things happened to them as an example, but they were written down for our instruction, on whom the end of the ages has come. We find great examples to learn from in the stories of the Old Testament.
Paul tells Timothy that the Old Testament is very useful.
But as for you, continue in what you have learned and have firmly believed, knowing from whom you learned it and how; from childhood you have been acquainted with the sacred writings, which are able to make you wise for salvation through faith in Christ Jesus. All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be competent, equipped for every good work. (2 Timothy 3:14-17)
What Paul says here applies to all Scripture, including the New Testament, but he was telling Timothy specifically about the Old Testament. Those were the only Scriptures that Timothy could have known from childhood. What does Paul say it is good for? Teaching, reproof, correction, training in righteousness, and equipping us for every good work. Does this mean that we can be fully equipped only with the Old Testament? No, or else there would have been no need for the New Testament to be written. But what it does mean is that we cannot be fully equipped without the Old Testament. We need the examples and teaching about God that is found there.
How can you understand the New Testament without knowing about the Old Testament? In the New Testament, there are constant references to stories that are recorded in the Old Testament. You cannot really understand Hebrews without studying the Old Testament. The same is true for several other books on the New Testament. We need to study the Old Testament today so that we will understand the New Testament.
Never try to make the Old Testament laws to be our laws. But understand that we can learn a lot from it. We get encouragement, hope, and instruction from our studies of the Old Testament. Do not neglect that part of your Bible. Study it.