The entire letter deals mainly with the infiltration of Judaizers in those communities who sought to impose the keeping of the Law - circumcision, observing the Sabbath, celebrating Feasts and Festivals, etc. - upon the early Christian church.
Paul opposes this completely referring in chapter 1 to his own "former life in Judaism" and how he "was advancing in Judaism beyond many of [his] own age..." but all of this was BEFORE he came to know Christ.
In chapter 2, Paul references his rebuke of Peter who, like himself and the other Jewish Christians in Antioch, was behaving like a Gentile - which was acceptable and right. But when the Jewish Christians from James [in Jerusalem] came to visit, Peter suddenly began to distance himself from the Gentiles [as if they were unclean] and his behavior began to affect the other Jewish Christians in Antioch to do the same. Paul calls out this hypocrisy and reminds Peter, and all the other Jewish Christians, that "by works of the law no one is justified" [v.16]
Finally, in chapter 3 Paul explains that the promises made to Abraham were not to every Jewish person but only to Christ and that anyone who is in Christ is an offspring of Abraham and an heir according to the promise.