1) Paul is defending his gospel of justification by faith alone to the Galatians who had begun following a false gospel of works righteousness.
2) He appeals to Scripture, particularly the Old Testament, to show that salvation has always been by faith as demonstrated through Abraham who was justified before God by faith, not works.
3) Christ redeemed believers from the curse of failing to obey the law by becoming a curse himself, dying on the cross to pay the penalty for sin so that righteousness can be credited to those who believe by faith.