what good is the old testament?

Top 3 Reasons to Read the Old Testament:

(1) It is impossible to truly understand Jesus without it.
(2) It is impossible to truly understand the New Testament without it.
(3) It is impossible to truly understand our identity as Christians without it.

These are not the only reasons.  But they are enough to make my point.

John Bright argues the same thing in his book, The Kingdom of God, where he says it is impossible to understand the New Testament apart from the Old because “Christ has come to make actual what the Old Testament hoped for, not to destroy it and replace it with a new and better faith” (193). This is bad news for the church today because it has long since forgotten the Old Testament. Bright laments the “widespread biblical illiteracy” that characterizes our generation of Christians. And he points out the danger of reading only the New Testament because it results in a superficial understanding of the Bible. Bright uses a building to illustrate his point: “If the Old Testament be a building without a roof [i.e. because its hopes are yet unfulfilled], the New Testament alone may be very like a roof without a building—and that is a structure very hard to comprehend and very hard to hold up!” (192-193)

A wonderful example of this is the book of 1 Peter. If you crossed out all of the Old Testament quotations and illusions in that short NT book there would be hardly a full sentence left over!  Peter bases his entire message for how the church should behave on their identity as the people of God (1 Peter 2:9-10) and in the example of Jesus (1 Peter 2:21-25).  How does that relate to the OT?

The ‘identity’ language is taken straight from Moses’ words to the Israelites in Ex 19:5-6 (which are later repeated in Deuteronomy 7:6 and several times later) where he calls them a “holy nation,” a “kingdom of priests,” and "God's treasured possession." Peter was not just making up nice things to say about NT believers.  What Moses said about Israel, Peter applies to a mixed church of both Jews and Gentiles! This radical shift is made possible by their faith in Jesus, the Messiah, who is the only true Israelite (because of his faith and perfect obedience). As Jesus took on the role to which Israel was called in Isaiah (Isa 42:1- as a light to the nations), he became the Servant whose suffering brought healing to the nations (52:13-53:12). Now the church, in following this suffering Servant, can expect also to suffer. Understanding the background for Peter’s words brings the rich depths of his theology into view. Without it we may scratch our heads and wonder why so many different metaphors are crowded into such a short letter.

This is just one example of why knowing the Old Testament is so vital.  It would not be exaggerating to say that it is impossible to accurately and fully understand the New Testament without a basic understanding of the Hebrew Scriptures.  I challenge you to read 1 Peter and underline everything that you think you've read in the Old Testament before.  I'm going to try it one of these days.  I've heard there are as many as 40 quotations and allusions there.  (And if you're really up for a challenge, try Romans 9-11, where there are reported to be no less than 100 quotes and allusions in just 3 chapters!)