What does Galatians 3:17 mean?
Paul is building the legal case for why those who are in Christ by faith, even non-Jewish people (Gentiles), become descendants of Abraham and receive the blessings promised to him. In the previous verse, Paul said that even a human covenant, or "will," cannot be changed once it has been ratified. When that kind of document was officially approved by all parties and legally filed, it was irrevocable.Now Paul clarifies the point of his illustration. God made a covenant with Abraham. That covenant was a one-way promise by God to bless Abraham and his offspring in specific ways. The Judaizers were apparently telling people that the law replaced God's covenant promises to Abraham (Galatians 2:4). Paul soundly rejects that idea. Although the law was instituted 430 years later, under Moses, it did not void out God's previous promises to Abraham. Those remain in effect, because God's covenant cannot be changed.
In the last verse, Paul made a point of indicating that this promise was given to Abraham's "singular" offspring, not a "plural" group or collection of many. Judaism had always believed that the promises given to Abraham would one day be fulfilled in a single person—a Messiah.
Jesus, as Abraham's ultimate offspring, received those covenant promises made to Abraham. That's why all who trust in Christ's death for their sin—placing their own lives "in Christ"—also receive the blessings promised to Abraham's offspring. We receive whatever Jesus is entitled to because we have been given credit for His sinless life, and He has taken the penalty for our sinfulness.