Why is it important for Christians today to understand the Jewish scriptures of the Old Testament?
Try for a moment to pretend that you no nothing of the Old Testament. You sit down and read about the destruction of Jerusalem in Luke 17: "In that day, he who is on the housetop, and his goods are in the house, let him not come down to take them away. And likewise the one who is in the field, let him not turn back. Remember Lot's wife. Whoever seeks to save his life will lose it, and whoever loses his life will preserve it" (Luke 17:31-33). Now, explain the meaning of the warning, "Remember Lot's wife" without reference to the Old Testament.
Next, go to Hebrews 11 which contains a list of the heroes of faith. Explain why these people were chosen and how they demonstrated faith without reference to the Old Testament.
Finally, start thumbing through your New Testament and look at all the quotes made from the Old Testament. Some are stand-alone quotes, but some are meant to trigger your memory of the context from which the quote is drawn. For example, as Jesus died on the cross, he refers to Psalms 22 by only quoting the first verse (Matthew 27:46; Mark 15:34). Can you possibly know what Jesus was inferring without reading the remainder of the psalm?
In explaining why Christians need to be concerned about others, Paul offers the example of Christ: "For even Christ did not please Himself; but as it is written, "The reproaches of those who reproached You fell on Me." For whatever things were written before were written for our learning, that we through the patience and comfort of the Scriptures might have hope" (Romans 15:3-4). Here insight to Christ's character was revealed in the Old Testament and Paul tells us that by learning the Old Testament we can have hope by learning patience and comfort from its words.
As an example, could you really appreciate the certainty of God's nature in keeping His word without seeing the many examples of His steadfastness over thousands of years? Could you appreciate the inspiration of the Scriptures if you didn't see all the prophecies being fulfilled? Think of the proof Christ offered the Jews: "You search the Scriptures, for in them you think you have eternal life; and these are they which testify of Me" (John 5:39).
The Old Testament also servers as counter-examples. After citing several examples of failures of the Israelites, Paul said, "Now these things became our examples, to the intent that we should not lust after evil things as they also lusted" (I Corinthians 10:6). After giving a few more examples, Paul stated, "Now all these things happened to them as examples, and they were written for our admonition, upon whom the ends of the ages have come" (I Corinthians 10:11).
Full understanding of the New Testament cannot be had without understanding the Old Testament.