What does Galatians 3:20 mean?

In the previous verse, Paul began to answer the question "why then the law?" In other words, if the law cannot save us from sin, as the Judaizers were teaching (Galatians 2:4), what was the point of it? Paul has said that the law was given to Israel for two reasons, so far. One is that the law defined certain sins and showed us that we are sinful. Two, the law was given for a season, from 430 years after God's promises to Abraham and his offspring, until the arrival of his ultimate offspring, Jesus, sent to receive those promises on our behalf.

Then Paul added that the law was put in place through angels and by an intermediary, from the Greek term mesitou, also translated as "mediator." In other words, the law was a covenant between two parties, God and Israel. God was represented in this agreement by angels. Israel was represented by Moses. The covenant agreement was this: If Israel would keep God's commands, He would bless them. If the people disobeyed, God would curse or punish them. It was a two-way covenant.

God, however, is "one," Paul concludes. He does not require human beings to be involved in order to make a covenant. We call this a unilateral covenant or promise. God gave promises to Abraham and his offspring without demanding anything in return. Abraham simply believed, and God gave. Jesus is "the offspring" who received what was promised to Abraham's descendants. Those who are "in Christ" receive those promises, as well, without need of help from the law.