The law was “added” (v.19cf.Deut.5:17) to the covenant God made with Abraham that promised him and his seed eternal life in exchange for their faith. But Paul just said you can’t add to a covenant (3:15), so God didn’t add the law to the terms of salvation in the covenant. He added it “because of transgressions” (v.19) i.e., to make their transgressions worse (Rom.5:20;7:13), to make them see their need for God to save them, “till the seed came to whom the promises were made”—i.e., till Christ came to save them (Gal.3:16).
The law was “ordained” (3:19), a word which means to establish, as it does in Habakkuk 1:12. God established the law (Ps.78:4,5) “by angels” (Gal.3:23), a reference to Deuteronomy 33:2. In so doing, God was abiding by the law He gave Israel. The law demanded two or three witnesses to “establish” a matter (Deut.19:15), so to establish something as monumentally and eternally important as the law, God used thousands of witnesses, as Moses said (Deut.33:2).
The word “mediator” comes from the word median, which means middle. A mediator is one who stands in the middle of two parties who are in disagreement to try to settle the problem. The first problem the Jews had with God was their fear of Him (Ex.20:13-19,21). Moses settled that by standing between them, as he later said when describing Exodus 20 (Deut.5:5), and speaking God’s law to them without all the scary lightning and thunder. God understood their fear, and said they “spoke well” in asking for a mediator (Deut. 5:24), and chose Moses to mediate the problem of their fear, for he was “meek” (Num.12:3), the very opposite of scary!
God said the Jews also spoke well when they said they’d do all that He said in His law always (Deut.5:27-29). But he knew they couldn’t, so He longed that they might have a heart to do it. But for that problem they’d need another mediator, so God promised them another one in response to their request for a mediator that day (Deut.18:15-19). That’s a promise of Christ, who was meek like Moses (Mt.11:25,28-30). Moses was a type of Christ in many ways.
A “yoke” (Mt.11:29,30) is a piece of wood that stands be-tween oxen to get them pulling in the same direction, on the same team, as it were—like a mediator! The Lord’s yoke was “easy” and “light” because His yoke was just to believe on Him. Men needed a light yoke because the yoke of the law of Moses was hard and heavy (Acts 15:10), because men couldn’t do it. But anybody and everybody can believe!
Moses did two things that pictured how Christ made His yoke light and easy. He knew the Jews couldn’t keep the law like they said they would, so he sacrificed animals and put half on God’s altar, and half on the Jews (Ex.24:3-8). That pictures how the Lord’s blood satisfied God’s righteousness and atoned for our sins. Moses also prayed for his people, as the Lord does for us (Rom.8:34).
The three persons of the Godhead are one (cf.Jo.10:30), so they don’t need a mediator to settle any disputes (Gal.3:20). After giving the Jews the law in Deuteronomy 5, God told them “God is one” (3:21cf.Deut.6:4), implying that if they kept His law they could be one with God. God knew they couldn’t, so before even giving the law He made Abraham one with Himself by making him righteous by faith (Gen.15: 5, 6). We know God gave him His righteousness, as he does us (IICor.5:21), for Paul uses him as an example in speaking of how we were given righteousness by faith in Romans 4.
That means the law wasn’t “against” God’s promise to give Abraham salvation by faith (Gal.3:21). Adam proved men can’t even keep one law, let alone the 613 commandments in the law, so God can’t save us by laws. He gave it to conclude us under sin (Gal.3:22). He saves us “by the faith of Christ” (3:22), i.e., by the Christ who faithfully kept the law we couldn’t keep, and died a sacrificial death on our behalf.