Galatians is written to people who are being told that they must do something by their own merits to join, or be identified with, Israel.  Obviously they’ve seen something in their belief in the God of Abraham, Isaac, and Jacob that tells them they are connected with the natural seed line to have opened themselves up to this type of deceptive reasoning.  Paul is emphatically explaining that they are connected, through their faith in Yeshua (Gal. 3:29)!