Who is under the law?

All are accountable to God.

Romans 3;19

Those under the law are every unredeemed human being.  Jews received the written law through Moses (Romans 3:2), and Gentiles have the works of the law written on their hearts (Romans 2:15), so that both groups are accountable to God.  There is no defense against the guilty verdict God pronounces on the entire human race.

Commentary from the MacArthur study Bible, notes for Romans 3:19.