What does Galatians 3:29 mean?

Paul has just described the incredible position of his Christian readers in Galatia. They, like all saved Christians, are full children of the Almighty God. They have been baptized into God's Spirit. They have put on Christ and are so completely covered by Him as to be fully identified by the Father with Christ. Finally, in Christ, they have equal value in the eyes of God, no matter their ethnicity, gender, or social status.

Now he brings this part of his case to a close by bringing it back to his earlier statement that believers in Jesus are also Abraham's offspring and heirs of the promises given to him by God (Galatians 3:7–9). More specifically, Christ is the ultimate offspring of Abraham. All the promises of God made to Abraham and his offspring flow to Christ. That's why, by being in Christ, covered by Him, identified with Him, we also become Abraham's children and heirs.

Our position before God, Paul insists, has nothing to do with following the law and everything to do with being in Christ. We have everything in Him, and nothing without Him.