He pointed out two contrasts between Ishmael and Isaac. First, Ishmael"s mother was a slave, but Isaac"s mother was free. These conditions affected the status of their sons in Abraham"s household. Second, Ishmael was born naturally, but Isaac was born supernaturally in fulfillment of God"s promise. [source][source][source]
"In the scriptural record of the birth of these two sons of Abraham Paul recognizes the same opposition between reliance on self ("according to the flesh") and reliance on God ("through promise") as exists between those who would be justified by legal works and those who are justified by faith." [1][source]