Galatians 4:21-31

v21-27: Abraham's son Ishmael was born to a slave woman (Hagar) by the ordinary course of nature, whereas Isaac was born to a free woman (Sarah) according to God's promise because "in the course of nature there was no reason to expect Sarah to have a son." These two sons are types.  Ishmael represents those who are attempting to be justified by their works and are in slavery to the law, where Isaac represents those who's faith is in Christ alone, and have been set free "from the curse of the moral law and from the slavery of the ceremonial law."

v28-31: "If we accept Christ, depend on him, and look for justification and salvation only through him, then, just as in this way we became the spiritual offspring of Abraham - though not his natural offspring - so we are also entitled to the promised inheritance and privilege of sharing in its blessings."