Because there has been so much confusion about the law, Paul goes on to explain its purpose and its relationship to the grace we receive through faith in Christ. He points out that God made a covenant with Abraham that he would be a blessing to all nations through his offspring. While even in Greek (and I guess Hebrew too?) “offspring” can be used as a collective noun, so that its singular form can refer to a group of offspring, Paul emphasizes that the blessing God promised came through only one of Abraham’s offspring, the Messiah, Jesus. Because the covenant and the promise were established before the law was given, then the fulfillment of the promise is not dependent on the law either. This is what the Pharisees and the Judaizers had gotten wrong. They were trying to attain God’s blessing by keeping the law as well as they could. But if that were possible, then the blessing wouldn’t come through the promise, and that means God would break His word.
That doesn’t mean the law is useless, however. Nor is it opposed to the covenant promise. Instead, they are complimentary because the law points out the need for the promise by exposing sin and revealing to us how big a deal it is to God. It also functioned as a guardian until the fullness of the promise was revealed. A guardian was a household slave who took care of the master’s children. Their job was to teach and correct, even doling out punishments for misbehavior. Additionally, they would act as chaperones and seek to guard against bad influences. Just as an heir is no longer under the guardian’s authority when he reaches majority, we are no longer under the ceremonial law which caused Israel to be distinct and separate from its neighbors. Now, our common faith in Christ allows us all to be unified with Him, regardless of any physical or social status we happen to have.
It is wondrous that You would adopt us as co-heirs with Christ, after all we have done before You rescued us.