Isaac had now laid aside all thoughts of going to Egypt, and, in obedience to the heavenly vision, sets up his staff in Gerar, the country in which he was born (Gen 26:6), yet there he enters into temptation, the same temptation that his good father had been once and again surprised and overcome by, namely, to deny his wife, and to give out that she was his sister. Observe,
I. How he sinned, Gen 26:7. Because his wife was handsome, he fancied the Philistines would find some way or other to take him off, that some of them might marry her; and therefore she must pass for his sister. It is an unaccountable thing that both these great and good men should be guilty of so strange a piece of dissimulation, by which they so much exposed both their own and their wives' reputation. But we see, 1. That very good men have sometimes been guilty of very great faults and follies. Let those therefore that stand take heed lest they fall, and those that have fallen not despair of being helped up again. 2. That there is an aptness in us to imitate even the weaknesses and infirmities of those we have a value for. We have need therefore to keep our foot, lest, while we aim to tread in the steps of good men, we sometimes tread in their by-steps.
II. How he was detected, and the cheat discovered, by the king himself. Abimelech (not the same that was in Abraham's days, ch. 20, for this was nearly 100 years after that, but this was the common name of the Philistine kings, as Caesar of the Roman emperors) saw Isaac more familiar and pleasant with Rebekah than he knew he would be with his sister (Gen 26:8): he saw him sporting with her, or laughing; it is the same word with that from which Isaac has his name. He was rejoicing with the wife of his youth, Pro 5:18. It becomes those in that relation to be pleasant with one another, as those that are pleased with one another. Nowhere may a man more allow himself to be innocently merry than with his own wife and children. Abimelech charged him with the fraud (Gen 26:9), showed him how frivolous his excuse was and what might have been the bad consequences of it (Gen 26:10), and then, to convince him how groundless and unjust his jealousy of them was, took him and his family under his particular protection, forbidding any injury to be done to him or his wife upon pain of death, Gen 26:11. Note, 1. A lying tongue is but for a moment. Truth is the daughter of time; and, in time, it will out. 2. One sin is often the inlet to many, and therefore the beginnings of sin ought to be avoided. 3. The sins of professors shame them before those that are without. 4. God can make those that are incensed against his people, though there may be some colour of cause for it, to know that it is at their peril if they do them any hurt. See Psa 105:14, Psa 105:15.

He said, She is my sister - It is very strange that in the same place, and in similar circumstances, Isaac should have denied his wife, precisely as his father had done before him! It is natural to ask, Did Abraham never mention this circumstance to his son? Probably be did not, as he was justly ashamed of his weakness on the occasion - the only blot in his character; the son, therefore, not being forewarned, was not armed against the temptation. It may not be well in general for parents to tell their children of their former failings or vices, as this might lessen their authority or respect, and the children might make a bad use of it in extenuation of their own sins. But there are certain cases, which, from the nature of their circumstances, may often occur, where a candid acknowledgment, with suitable advice, may prevent those children from repeating the evil; but this should be done with great delicacy and caution, lest even the advice itself should serve as an incentive to the evil. I had not known lust, says St. Paul, if the law had not said, Thou shalt not covet. Isaac could not say of Rebekah, as Abraham had done of Sarah, She is my sister; in the case of Abraham this was literally true; it was not so in the case of Isaac, for Rebekah was only his cousin. Besides, though relatives, in the Jewish forms of speaking, are often called brothers and sisters, and the thing may be perfectly proper when this use of the terms is generally known and allowed, yet nothing of this kind can be pleaded here in behalf of Isaac; for he intended that the Gerarites should understand him in the proper sense of the term, and consequently have no suspicion that she was his wife. We have already seen that the proper definition of a lie is any word spoken with the intention to deceive. See Gen 20:12.

And it came to pass, when he had been there a long time,.... Perhaps some years: for though it is in the original, "when days were prolonged to him there" (b); yet days are sometimes put for years:

that Abimelech king of the Philistines looked out at a window; in his own palace, near to which, in all likelihood, were the apartments that Isaac and Rebekah dwelt in; and this he did accidentally, and not out of curiosity, or with any intention to observe or pry into the behaviour and conduct of these two persons one towards the other:

and saw, and, behold, Isaac was sporting with Rebekah his wife; laughing and joking with her, which by his motions and gestures, and the airs and freedoms he took, Abimelech could perceive were such as were not usual between brothers and sisters, though honest and lawful between man and wife; such as embracing her in his arms, and frequently kissing her, and the like.

(b) "cum prolongati essent ei ibi dies", Pagninus, Montanus, Schmidt.