According to the story told in Gen 26:1-11, Isaac went with his wife Rebecca to dwell for a while in Gerar. Afraid that the locals might kill him to gain his beautiful wife, Isaac passed her off as his sister. One day, however, Abimelech the king, seeing Isaac playing with Rebecca in a manner that a man should do only with his wife, discovers the truth about their relationship. (Gerhard von Rad comments: “Details are left to the reader’s imagination; the narrator veils it with an etymological allusion to Isaac’s name (yishaq-sahaq), ‘jest’” (Genesis, p. 266). Challenged by a Manichean to make sense of the tale, Augustine looks for and finds its messianic symbolism.
Some inquirer may ask what mystery is signified when the foreign king discovers Rebecca to be the wife of Isaac because he sees him playing with her. He would not have known this if he had not seen Isaac playing with Rebecca in a way that would have been improper with a woman not his wife. When holy married men act in this way, they don’t do so foolishly, but knowingly; in a way they’re lowering themselves to the weakness of the female sex as they say and do gentle and playful things, not weakening but tempering their manliness. But such behavior towards any woman except one’s wife would be disgraceful. I say this, which is a matter of proper conduct, lest someone hard and insensible find fault with that holy man because he was playing with his wife. Such inhuman human beings, if they see some serious man playfully chattering with his little children, pleasing and nourishing their nascent minds by his kind and attentive ease with them, they think he’s being silly. They forget how they themselves grew up and are ungrateful for it.
Now what it means for the mystery of Christ and the Church that so great a patriarch was playing with his wife and that their married relationship was discovered in this way anyone can see who ... carefully studies in the Scriptures the secret of her Bridegroom. There he will find that for a time he concealed in the form of a slave the majesty by which he was in the form of God equal to his Father (Ph 2:6-7) so that feeble humanity might become capable of him and that in the same way he might accommodate himself to his wife. There’s nothing absurd, then, in fact it was a suitable anticipation of the future, that a prophet of God should indulge in fleshly play to gain his wife’s affection, since the very Word of God became flesh in order to dwell in us (Contra Faustum, 22, 46; PL 42).