|Matthew Henry's Concise Commentary|
26:34,35 Esau was foolish in marrying two wives together, and still more in marrying Canaanites, strangers to the blessing of Abraham, and subject to the curse of Noah. It grieved his parents that he married without their advice and consent. It grieved them that he married among those who had no religion. Children have little reason to expect God's blessing who do that which is a grief of mind to good parents.
Verse 35. - Which were a grief of mind (literally, bitterness of spirit) unto Isaac and to Rebekah - possibly because of their personal characters, but chiefly because of their Canaanitish descent, and because in marrying them Esau had not only violated the Divine law which forbade polygamy, but also evinced an utterly irreligious and unspiritual disposition.
Gill's Exposition of the Entire Bible
Which were a grief of mind unto Isaac, and to Rebekah. The marriage of them itself was a trouble to them, it being contrary to their will that any of their children should marry with the Canaanites, and those the worst sort of them, the Hittites; it having been the care of Abraham, the father of Isaac, that his son should not marry with them, and laid a strict injunction on his servant not to take a wife for his son from among them; and which was an example to be followed in later times, and which Esau very likely was not ignorant of: and besides this, the women themselves he took for wives were very disagreeable on all accounts, partly because of their religion, being idolaters, and partly by reason of their temper and behaviour, being proud, haughty, and disobedient; as all the three Targums intimate.
Genesis 26:35 Parallel Commentaries
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