With the understanding of our New Covenant in Christ, many people question the role of the Old Testament in Christianity. 
The answer to this pertinent question can be captured in Paul’s challenge to Timothy,

[do] your best to present yourself to God as one approved by him, a worker who has no need to be ashamed, rightly explaining the word of truth. (2 Tim. 2:15)

At the time of writing, Christianity was still at its formative state, hence there was no text known as the New Testament. Clearly this admonition was for the study of the Torah, Mishnah, Midrash, and other classical Jewish writings. 
Paul himself had studied under Gamaliel, a known Rabbi with intellectual lineage to back his teaching and worldview. With a view to assert his authority, Paul gives us a preview of his credentials when he notes,

“I am a Jew, born in Tarsus of Cilicia,but brought up in this city. I studied under Gamaliel and was thoroughly trained in the law of our ancestors. I was just as zealous for God as any of you are today.” (Acts 22:3)

Paul, and by proxy Timothy, were both baptised or filled with the Holy Spirit; yet he exhorted Timothy to study these texts. 

Dear friends, Paul was passing on old Jewish culture or tradition, in this case: lineage of knowledge or philosophy, and excellence in vocation. For example, like Jesus, Paul’s studied the equivalent of a degree in Theology; while Gamaliel would have stood on platforms as a Professor of Theology. 
In fact when the gospel writer asserts that Jesus,  

“taught as one who had authority, and not as their teachers of the law.” (Matthew 7:29); 

they were highlighting the fact that he was doing something unique with the text; in other words, shifting from his lineage. The same is done about John the Baptist when the writer notes that ‘he shall be called John’, and that he was, ‘in the wilderness until the day of his showing to Israel’.

To this end the Old Testament serves the following primary purpose:
1. It provides us with materials to explore the rich history, culture, context and worldview of the Jews, for the purpose of understanding and teaching 

1. It shows us the theological progression of the Jews, and the intersection by Christ, in his effort to Reconcile the ‘Gentiles’ into a New Covenant

2. It gives us an understanding of what the New Testament writers were trying to achieve in their effort to bridge the gap between Judaism and what we know today as ‘Christianity’ 

3. It gives a blue print for the Christian faith. Which can be best described as a universal form of Judaism whose vision is for a kingdom bound by a common faith and destiny that is centred around Christ, as to a kingdom bound by heritage.
   Father, enlarge our hearts. Give us a vision for humanity. We ask through you son Jesus Christ. 

To a blessed week!