John Gills Exposition of the Bible Commentary - Genesis 17:19
And God said, Sarah thy wife shall bear thee a son indeed ,.... This is repeated for the confirmation of it, and thus expressed to remove all doubt about it, if any there were, that hung upon Abraham's mind; as well as to let him know that the promise of a son by Sarah was not to be superseded by his prayer for Ishmael, for whom he might have a greater flow of natural affection than for his unborn son, in whom his seed should be called: and thou shall call his name Isaac ; which... read more
Matthew Henry's Complete Commentary - Genesis 17:15-22
Here is, I. The promise made to Abraham of a son by Sarai, that son in whom the promise made to him should be fulfilled, that he should be the father of many nations; for she also shall be a mother of nations, and kings of people shall be of her, Gen. 17:16. Note, 1. God reveals the purposes of his good-will to his people by degrees. God had told Abraham long before that he should have a son, but never till now that he should have a son by Sarai. 2. The blessing of the Lord makes fruitful, and... read more