Wherefore then [serveth] the law? It was added because of transgressions, till the seed should come to whom the promise was made; [and it was] ordained by angels in the hand of a mediator.
Note 1 at Ga 3:19: Paul had already proven that the Law was not given to bring justification (see notes 3-4 at Ro 3:19 and note 14 at Ro 3:31). Then why was it given? It was added (this verse) to the covenant of promise (Ga 3:18) to magnify our sin (see note 5 at Ro 7:11) and therefore remove any false ideas we might have of saving ourselves. It ruled out the possibility of self-salvation and made us ready for the covenant of grace (Ga 3:24). It was to remain valid and in force until the coming of the one true descendant of Abraham; i.e., Christ (Ga 3:16).Note 2 at Ga 3:19: The use of the words "added" and "till" are very significant. The Law was neither God’s first nor His primary way of dealing with mankind. The covenant of grace, or promise (Ga 3:18), had already been in effect for 430 years before God gave the Law through Moses (Ga 3:17, see note 2 at Ro 4:10) and therefore had greater authority.
The word "till" is also used to show that the Law was only temporary until Christ came. When Christ came, He put an end to the Law for righteousness (Ro 10:4). Those who advocate the keeping of the Law for the purpose of right standing with God are going back to an Old Testament system of law that has been abolished (2Co 3:13 and Eph 2:15) and are making the work of Christ void in their lives (Ga 2:21).
Note 3 at Ga 3:19: The word "mediator" literally means to be in the middle or midst. It is "'one who mediates' between two parties with a view to producing peace" (Vine's Expository Dictionary). Moses was the mediator God used to make the Old Testament covenant of Law between Himself and the Jews. In the New Testament, Jesus is the mediator between God and mankind (1Ti 2:5, see note 4 at Ga 3:20).