What is the Purpose of the Law? (Part 1)
Galatians

The Law was added because of transgressions (Gal a)

Transgressions (parabasis) = stepping aside, overstepping, stepping over the line, breaking the law.
The Law was added - it did not replace or invalidate the promise.
The Law was added because of "law-breaking" - to show them the depth of their transgressions. The purpose of the Law was to point out how sinful the heart of mankind really is, and to show us our guilt before God. The Law is like a "Sin Meter" that squawks when we step over the line of righteousness. It shows us we are unrighteous.
Thus the Law motivates us to seek the deliverance or rescue that God promises to us in Jesus Christ.

God gave the Law to the nation of Israel through two mediators:
1. Angels were used to deliver the Law to Moses (see Acts ; Hebrews ).
2. Moses was used to deliver the Law to the people of Israel.
A mediator acts as a "go-between" when there are two parties or groups.
But the promises God gave to us through Abraham were given by God Himself without any mediator. When God gave the promises to Abraham, there was only one party to the covenant - "God is one" - remember that Abraham was asleep! (Genesis , )
By exalting the Law, you exalt Moses and angels. Moses was great, and Angels were great, but Paul warns the Galatians not to exalt Moses or angels over God Himself. Exalt God alone!
UNTIL = the Law was only temporary. It was meant to be in force only until "THE SEED" (Gal. ) came. After this, the Law was finished.

The Law does not contradict the promises of God. If it were possible for mankind to be justified before God by obedience to a law, then this was the Law that could have done it! It was as good a Law as could have been made, and it accomplished the purpose it was intended to fulfill.
There is no law that can impart life. It was never possible to be justified before God by obeying any law. The problem with mankind is not what we do but what we are by nature. At the Fall our very nature was twisted or corrupted. It is what we are that offends God.
Shut up (sunkleio) = a strong term meaning "to lock up securely" or "to enclose on all sides with no means of escape."
The function of the Law is to show that all people are sinners, and to close off all other alternatives for being justified. God's plan of justification by faith in Christ then becomes the only way that sinful mankind can be justified!
The Law arrests us, and imprisons us, and proves our guilt before God. The Law shuts us in, captures us, and leaves no way out. But that is when the promise can come to our rescue!