The function of the Law

But when Jesus heard that, he said to them, They that be whole need not a physician, but they that are sick.

Matthew 9:12

"Sinners that think they need no physician will not endure the healer's hand. The Law is therefore necessary to give knowledge of sin so that proud man, who thought he was whole, may be humbled by the discovery of his own great wickedness, and sigh and pant after the grace that is set forth in Christ"

Martin Luther

Wherefore if ye be dead with Christ from the rudiments of the world, why, as though living in the world, are ye subject to ordinances, Touch not; taste not; handle not...

Colossians 2:20-21

Some may wonder whether using the Law in evangelism produces legalism.When the Law is used to show a sinner that sin is "exceedingly sinful" - that nothing can commend him to God - he clings to the cross knowing that he is saved by grace and grace alone. This knowledge gives the Christian the understanding that even after the lifetime of good works, fasting, praying, seeking the lost, ets., his "works" don't commend him to God - he is still saved by grace alone.
However, when the Law isn't used befor the cross, and the sinner simply makes a "decision for Christ," he comes with the lack of understanding about the true nature of sin. After his commitment, he thinks that his good works, his fasting, praying, evangelism,ets., commend him to God. He is the one who thinks that what he eats, wears, and what he does become relevant to his salvation. He is the one who is liable to say "touch not, taste not, handle not"- the one who becomes "legalistic."
But using the Law in evangelism befor the cross liberates a new convert from legalism.

"Just as the world was not ready for the New Testament before it received the Old, just as the Jews were not prepared for the ministry of Christ until John the Baptist had gone before Him with his claimant call to repentance, so the unsaved are in no condition today for the gospel till the Law be applied to their hearts, for 'by the Law is the knowledge of sin.' It is the waste of time to sow seed on ground which has never been ploughed or spaded! To present the vicarious sacrifice of Christ to those whose dominant passion is to take fill of sin, is to give that which is holy to the dogs."

A.W. Pink

"God, being a perfect God, had to give a perfect Law, and the Law was given not to save men, but to measure them. I want you to understand this clearly, because I believe handreds and thousands stumble at this point. They try to save themselves by trying to keep the Law; but it was never meant for men to save themselves by."

D.L.Moody