Complete Commentary on the Whole Bible, by Matthew Henry, at sacred-texts.com
(Gen 20:1-8) Abraham's sojourn at Gerar, Sarah is taken by Abimelech.
(Gen 20:9-13) Abimelech's rebuke to Abraham.
(Gen 20:14-18) Abimelech restores Sarah.
Crooked policy will not prosper: it brings ourselves and others into danger. God gives Abimelech notice of his danger of sin, and his danger of death for his sin. Every wilful sinner is a dead man, but Abimelech pleads ignorance. If our consciences witness, that, however we may have been cheated into a snare, we have not knowingly sinned against God, it will be our rejoicing in the day of evil. It is matter of comfort to those who are honest, that God knows their honesty, and will acknowledge it. It is a great mercy to be hindered from committing sin; of this God must have the glory. But if we have ignorantly done wrong, that will not excuse us, if we knowingly persist in it. He that does wrong, whoever he is, prince or peasant, shall certainly receive for the wrong which he has done, unless he repent, and, if possible, make restitution.
See here much to blame, even in the father of the faithful. Mark his distrust of God, his undue care about life, his intent to deceive. He also threw temptation in the way of others, caused affliction to them, exposed himself and Sarah to just rebukes, and yet attempted an excuse. These things are written for our warning, not for us to imitate. Even Abraham hath not whereof to glory. He cannot be justified by his works, but must be indebted for justification, to that righteousness which is upon all and unto all them that believe. We must not condemn all as hypocrites who fall into sin, if they do not continue in it. But let the unhumbled and impenitent take heed that they do not sin on, thinking that grace may abound. Abimelech, being warned of God, takes the warning; and being truly afraid of sin and its consequences, he rose early to pursue the directions given him.
We often trouble ourselves, and even are led into temptation and sin, by groundless suspicions; and find the fear of God where we expected it not. Agreements to deceive generally end in shame and sorrow; and restraints from sin, though by suffering, should be thankfully acknowledged. Though the Lord rebuke, yet he will pardon and deliver his people, and he will give them favour in the sight of those with whom they sojourn; and overrule their infirmities, when they are humbled for them, so that they shall prove useful to themselves and others.