Is he the God ... - The Jews supposed that he was the God of their nation only, that they only were to be admitted to his favor. In these verses Paul showed that as all had alike sinned, Jews and Gentiles; and as the plan of salvation by faith was adapted to sinners, without any special reference to Jews; so God could show favors to all, and all might be admitted on the same terms to the benefits of the plan of salvation.
It is one God - The same God, there is but one, and his plan is equally suited to Jews and Gentiles.
The circumcision - Those who are circumcised - the Jews.
The uncircumcision - Gentiles; all who were not Jews.
By faith ...through faith - There is no difference in the meaning of these expressions. Both denote that faith is the instrumental cause of justification, or acceptance with God.