Verses 1-8, Justification is by God's grace thru faith alone.

Verse 9-12, Circumcision is not a basis of Justification seeing that Abraham was justified by faith before he was circumcised.

Verse 13-16, God's promise to Abraham did not come by keeping the law.

Verse 18-25, Summarizes Paul's argument by concluding Abraham was justified by his faith and not the law.