that Abraham had two sons,
not two sons only; for besides the two referred to, he had six more, ( Genesis 25:2 ) but it being only pertinent to the apostle's purpose to take notice of these two, he mentions no more, though he does not deny that he had any more. These two sons were Ishmael and Isaac:
the one by a bondmaid.
Ishmael was by Hagar, Sarah's servant, who represented the covenant the Jewish nation was under the bondage of.
The other by a free woman.
Isaac was by Sarah, Abraham's proper and lawful wife, who was mistress of the family, and represented in figure the covenant, and Gospel church state, and all believers, Gentiles as well as Jews, as under the liberty thereof.