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.

Isaac was sporting with Rebekah his wife - Whatever may be the precise meaning of the word, it evidently implies that there were liberties taken and freedom used on the occasion, which were not lawful but between man and wife.

And Abimelech said, what is this thou hast done unto us?.... By entertaining suspicions and jealousies of us as bad men, and by exposing us to the temptation of committing iniquity; why hast thou dealt thus with us, and what have we done, or is in our character, that thou shouldest act after this manner?

one of the people might lightly have lien with thy wife; it is much one or other had not before this time, not looking upon it criminal to have to do with a single woman, when they would not have meddled with another man's wife, Jarchi interprets this of Abimelech himself; and so the Targum of Jonathan, who perhaps had been thinking of taking her to his bed; and was "within a little" (c), as the word for "lightly" may be rendered, of accomplishing his design:

and thou shouldest have brought guiltiness upon us: been the occasion of their committing the sin of adultery, which was heinous in the eyes of Heathens, and of bringing on them the punishment due thereunto.

(c) "parum abfuit", Junius & Tremellius, Piscator, Schmidt.