19 Why then the law? It was added because of transgressions, until the offspring should come to whom the promise had been made, and it was put in place through angels by an intermediary. 20 Now an intermediary implies more than one, but God is one.
21 Is the law then contrary to the promises of God? Certainly not! For if a law had been given that could give life, then righteousness would indeed be by the law. 22 But the Scripture imprisoned everything under sin, so that the promise by faith in Jesus Christ might be given to those who believe.
23 Now before faith came, we were held captive under the law, imprisoned until the coming faith would be revealed. 24 So then, the law was our guardian until Christ came, in order that we might be justified by faith.
It will help us to a fuller understanding of this question if we look at it in this way: God's supreme purpose in making Himself known to man, and in giving the promise to Abraham, was to rid the world of sin. God knew that only through a costly act of redemption could this be effected, hence His purpose was to direct the attention of men to a Redeemer. But men do not look to a Redeemer until they feel their need of one, and only when they are conscious of the reality and seriousness of their sin are they likely to feel this need. But sin blinds. Man cannot know his sin by himself; he is sitting in darkness. How then can a Redeemer be seen to be relevant, if that from which men are to be redeemed is not seen as the horrible, tragic reality that it is? Therefore the need is that it should be revealed to men, and therefore the law was added because of transgressions; for by the law is the knowledge of sin (Romans 3:20). Hence the long centuries of preparation before the promise was fulfilled. The ministry of the law was designed to reveal man's sinfulness, and thereby develop the desire and expectation of a Redeemer. This in fact is the significance of Old Testament history. Down the ages Israel slowly learned the sinfulness of sin; hence, as time went on, the deepening yearning for One to come who would really deal with sin. And what is true of the divine plan of the ages is also true in the experience of individuals. This is the real place, function and significance of the law - not to provide a way of justification, but rather to bring us to a knowledge of sin, in order that we might seek the Saviour and find Him.