Galatians 3:26-29

You are all sons of God through faith in Christ Jesus,

(v26, NIV)

In the past little while, Paul has been using the word "we". However, now he switches to use the word "you" or "you all". He reminds them that they are sons of God. He is speaking to the Christians at Galatia, those who have already accepted Jesus as their lord and savior. They may have been mislead into believing that they are sons through the fact that they were Jews, but it is through faith in Jesus.

for all of you who were baptized into Christ have clothed yourselves with Christ. There is neither Jew nor Greek, slave nor free, male nor female, for you are all one in Christ Jesus.

(v27-28, NIV)

So they have already accepted Jesus and have been baptized into him. Having proclaimed Jesus as lord of their life, they must cloth themselves with Jesus. As representatives as Jesus, we must live worthy of the calling. Furthermore, we are no longer Jew or Greek. Instead, since we all have put on Christ, we are all therefore Christ's. There is no need for divisions within the church and these differences. There is unity in the body of Christ because Christ died so that we'd all be one together in unity.

If you belong to Christ, then you are Abraham's seed, and heirs according to the promise.

(v29, NIV)

If the Jews were arguing and differentiating themselves from others as Abraham's descendants, they could do so no longer. Or rather, they couldn't exclude other believers because all who are Christ's are Abraham's seed. There shouldn't be any more division within the body of Christ about this matter.