"nation" refers to the people in it. AT: "I will judge the people of the nation" (See: [[rc://en/ta/man/translate/figs-metonymy]])
# the nation that they serve
"the nation that they will serve"
# gave Abraham the covenant of circumcision
The Jews would have understood that this covenant required Abraham to circumcise the males of his family. AT: "made a covenant with Abraham to circumcise the males of his family" (See: [[rc://en/ta/man/translate/figs-explicit]])
# so Abraham became the father of Isaac
The story transitions to Abraham's descendants.
# Jacob the father
"Jacob became the father." Stephen shortened this. (See: [[rc://en/ta/man/translate/figs-ellipsis]])