In Galatians 4:22, Paul says that it is written that Abraham had two sons. But surely he had more than two sons! Paul never said Abraham had only two sons all together. I think he may be zooming in on these two sons because of the figurative application he was making. Also, perhaps Paul is saying that Abraham had two sons as it pertained to the promise. Abraham's later sons were not the result of a promise, but were sons from another woman who was not barren. Also, there was nothing miraculous about these births. Abraham had even more children by his concubines (Genesis 25:6). What do you say? -- Chad Garber
Exactly, Abraham had two sons as pertaining to the promise. Paul would have been well aware of Abraham's other sons. In a similar way, God's only Son is Jesus Christ--although he has others, too. Even in Genesis 22, Abraham (who by this time already has Ishmael) is told to take is "only" son and sacrifice him... This is normal biblical usage: to zero in on the subject at hand without any qualification, ignoring the irrelevant facts.