So is the Old Testament Law worthless? By no means!
The law serves a most necessary purpose.
They [unbelievers] will never accept grace
until they tremble before a just and holy Law.
I do not believe that any man can preach the gospel
who does not preach the law.
The purpose of the Law is to reveal to us our sin.
Romans 7:7-13 (ESV)
7 What then shall we say? That the law is sin? By no means! Yet if it had not been for the law, I would not have known sin. For I would not have known what it is to covet if the law had not said, “You shall not covet.”
8 But sin, seizing an opportunity through the commandment, produced in me all kinds of covetousness. For apart from the law, sin lies dead.
9 I was once alive apart from the law, but when the commandment came, sin came alive and I died.
10 The very commandment that promised life proved to be death to me.
11 For sin, seizing an opportunity through the commandment, deceived me and through it killed me.
12 So the law is holy, and the commandment is holy and righteous and good.
13 Did that which is good, then, bring death to me? By no means! It was sin, producing death in me through what is good, in order that sin might be shown to be sin, and through the commandment might become sinful beyond measure.
Romans 3:20 (ESV) For by works of the law no human being will be justified in his sight, since through the law comes knowledge of sin.
Romans 7:7 (ESV) What then shall we say? That the law is sin? By no means! Yet if it had not been for the law, I would not have known sin. For I would not have known what it is to covet if the law had not said, “You shall not covet.”
Romans 3:9-10 (ESV)
9 What then? Are we Jews any better off? No, not at all. For we have already charged that all, both Jews and Greeks, are under sin,
10 as it is written: “None is righteous, no, not one;
Galatians 2:19 (ESV) For through the law I died to the law, so that I might live to God.