If we belong to Christ, which we do, then we are Abraham’s seed (singular), which Galatians 3:16 made very clear was Christ ( Galatians 3:16). Paul was saying that God treats us as He treats Jesus, because we have become partakers with Him through faith. 1 Peter 4:1 tells us to arm ourselves with the same mind that Christ had. Philippians 2:5 says, “Let this mind be in you, which was also in Christ Jesus.” One of the keys to victorious Christian living is seeing ourselves “in Christ” with all the rights and privileges that Christ has (Romans 6:4-11).
Note 4 at Galatians 3:29:
Going back to Galatians 3:16, we discover that God’s promises were made to Abraham and his seed, Christ. Christ was participating in the covenant agreement on our behalf. Jesus had no need to be justified through faith. Therefore, as our representative, the promise was made to Him, and since we are “in Him,” we receive the benefits. We are also guaranteed that this covenant will not be voided by our performance,
because the promise was made to Christ, not to us (see note 5 at Galatians 3:20). We are beneficiaries of His goodness, not ours.
Bible commentary on Gal. 3:16