The word "it" refers to receiving what God had promised. Alternate translation: "it is by faith that we receive the promise" or "we receive the promise by faith"
Here "the promise may rest on grace" represents God giving what he promised because of his grace. Alternate translation: "so that what he promised might be a free gift" or "so that his promise would be because of his grace" (See: [[rc://en/ta/man/jit/figs-metaphor]])
Here the word "us" refers to Paul and includes all Jewish and non-Jewish believers in Christ. Abraham is the physical ancestor of the Jewish people, but he is also the spiritual father of those who have faith. (See: [[rc://en/ta/man/jit/figs-exclusive]])