Genesis 21:3

Abraham called the name of his son. — Attention has been called to the fact that we have here two things contrary to subsequent usage: for, first, the father names the child, and not the mother; and, secondly, he names him at his birth, instead of waiting until his circumcision. It might be enough to answer that the child was really named by God (Genesis 17:19), and that Abraham only acknowledges that the son born was the promised Isaac; but really, as we have seen before, there was as yet no settled rule as to either of these points.

Isaac. — This name not only recorded the fact of the laughter of the father (Genesis 17:17) and of the mother (Genesis 18:12), but was a standing memorial that Isaac’s birth was contrary to nature, and one of which the promise was provocative of ridicule in the sight even of his parents.