Genesis 17:19

Sarah thy wife God's answer in this verse is made to the utterance of Abraham's heart (Genesis 17:17), and not of his lips (Genesis 17:18).

thou shalt call his name Isaac R.V. marg. "From the Heb. word meaning to laugh." See Genesis 21:3. The name Isaac is here, and in 18 and 21, associated with "laughter." The word "he laughed," used in Genesis 17:17, has the same root letters (ṣḥq) as the name "Isaac." The name "laughter" will thus commemorate the involuntary doubt of Abraham (Genesis 17:17) to which St Paul refers (Romans 4:19), "without being weakened in faith he considered his own body now as good as dead (he being about a hundred years old) and the deadness of Sarah's womb."

Note that the father is commanded to give the name; see note on Genesis 5:3 (P).