This verse is an expansion of the previous one. Whereas in the previous verse the focus is upon the status of the mothers, here the focus is upon the sons. Ishmael is described as born in the usual way, which is, literally, “born after the flesh.” It is tempting to read ethical connotations into the expression, but what Paul has in mind is simply that Ishmael was born according to the natural process and not through divine intervention. When Hagar conceived a child by Abraham, she was obviously young and Abraham had not yet become too old a man to beget a child.
A literal translation of his son by the slave woman was born in the usual way may give rise to serious misunderstanding, particularly in contrast with the following clause. Born in the usual way may suggest that this was simply the normal kind of delivery, that is, with the head of the child coming out first, with the added inference that in some way or other the birth of Isaac was an unusual kind of birth, possibly a breech birth. What is involved here, of course, is not a reference to the manner of birth, but to the fact of coming into existence. One may therefore better translate the first part of this verse in some languages as “Abraham had a son by the slave woman in the same way that children are usually conceived and born.”
On the other hand, Isaac was born as a result of God’s promise (literally, “through promise”). Clearly this is a reference to the fact that when Isaac was conceived, Abraham and Sarah were already old and presumably incapable of having children and that therefore Isaac was conceived and born as a result of divine intervention. God promised Abraham and Sarah a son, and he fulfilled that promise in the birth of Isaac. Again the focus of attention is not on the manner of birth but on the conception. Therefore one may translate the second part of this verse as “Abraham had a son by his wife, who was not a slave, and this came about because God himself had promised such a son,” or “the fact that Abraham had a son by his wife, who was not a slave, happened as a result of what God had promised to Abraham.”
Quoted with permission from Arichea, Daniel C. and Nida, Eugene A. A Handbook on Paul’s Letter to the Galatians. (UBS Handbook Series). New York: UBS, 1976. For this and other handbooks for translators see here .