The name given to Abraham and his descendants, who later were more commonly called Israelites or Jews (Gen 14:13, 39:17). They were the ‘people group’ God chose to ‘be His own’ – He desired a distinct people who would love and obey Him and point others to Him. Although they largely failed to do this, Jesus Christ still came via this race and perfectly fulfilled the promise that the entire world would be blessed through them (Gen 12:1-3).
Their religion was that presented in the OT, especially the first five books (Genesis to Deuteronomy).
The NT book bearing this name was addressed to the Jewish Christians spread throughout the then known world.