Genesis 20:3

3.But God came to Abimelech in a dream by night. Here Moses shows that the Lord acted with such gentleness, that in punishing his servant, he yet, as a father, forgave him: just as he deals with us, so that, while chastising us with his rod, his mercy and his goodness far exceed his severity. Hence also we infer, that he takes greater care of the pious than carnal sense can understand; since he watches over them while they sleep. This also is to be carefully noticed; that however we may be despised by the worlds we are yet precious to him, since for our sake he reproves even kings, as it is written in Psalms 105:14. But as this subject was more fully discussed in the twelfth chapter, (Genesis 12:1) let the readers there seek what I now purposely omit. Whereas, God is said to have come, this is to be applied to the perception of the king, to whom undoubtedly the majesty of God was manifested; so that he might clearly perceive himself to be divinely reproved and not deluded with a vain spectre.

Behold, thou art but a dead man. Although God reproved king Abimelech, for the sake of Abraham, whom he covered with his special protection; he yet intends to show, generally, his high displeasure against adultery. And, in truth, here is no express mention of Abraham; but rather a general announcement is made, for the purpose of maintaining conjugal fidelity. ‘Thou shalt die, because thou hast seized upon a women who was joined to a husband.’ Let us therefore learn, that a precept was given in these words, to mankind, which forbids any one to touch his neighbor’s wife. And, truly, since nothing in the life of man is more sacred than marriage, it is not to be wondered at, that the Lord should require mutual fidelity to be cherished between husbands and wives and should declare that he will be the Avenger of it, as often as it is violated. He now addresses himself, indeed, only to one man; but the warning ought to sound in the ears of all, that adulterers — although they may exult with impunity for a time — shall yet feel that God, who presides over marriage, will take vengeance on them. (Hebrews 13:4.)