These are the generations of Isaac, Abraham’s son: Abraham fathered Isaac,
19. These are the generations of Isaac. Because what Moses has said concerning the Ishmaelites was incidental, he now returns to the principal subject of the history, for the purpose of describing the progress of the Church. And in the first place, he repeats that Isaac’s wife was taken from Mesopotamia.
We have here an account of the birth of Jacob and Esau, the twin sons of Isaac and Rebekah: their entrance into the world was (which is not usual) one of the most considerable parts of their story; nor is much related concerning Isaac but what had reference to his father while he lived and to his sons afterwards.
Commenting on Genesis 25:19-28
And these are the generations of Isaac, Abraham's son,.... Moses, having given the genealogy of Ishmael and his posterity, returns to Isaac, the other son of Abraham, with whom and his children the following part of his history is chiefly, if not altogether concerned: Abraham begat Isaac; for the further confirmation of his being his proper legitimate son this clause is added.