Genesis 16:2

2.That I may obtain children by her (384) This is a Hebrew phrase, which signifies to become a mother. Some however, expound the word as simply meaning, to have a son. And certainly בן (ben,) which, among the Hebrews, signifies son, corresponds with the verb here used. (385) But since sons are so called metaphorically as being the maintainers of the race, and thus building up the family, therefore the primary signification of the word is to be retained. But Sarai claims for herself by right of dominion, the child which Hagar shall bring forth: because handmaids do not bring forth for themselves, since they have not power over their own body. By first speaking to her husband, she does not barely allow of a concubine, who should be as a harlot; but introduces and obtrudes one. And hence it appears, that when persons are wiser in their own eyes than they ought to be, they easily fall into the snare of trying illicit means. The desire of Sarai proceeds from the zeal of faith; but because it is not so subjected to God as to wait his time, she immediately has recourse to polygamy, which is nothing else than the corruption of lawful marriage. Moreover, since Sarai, that holy woman, yet fanned in her husband the same flame of impatience with which she burned, we may hence learn, how diligently we ought to be on our guard, lest Satan should surprise us by any secret fraud. For not only does he induce wicked and ungodly men openly to oppose our faith; but sometimes, privately and by stealth, he assails us through the medium of good and simple men, that he may overcome us unawares. On every side, therefore, we must be on our guard against his wiles; lest by any means he should undermine us.

And Abram hearkened to the voice of Sarai. Truly the faith of Abram wavers, when he deviates from the word of God, and suffers himself to be borne away by the persuasion of his wife, to seek a remedy which was divinely prohibited. He, however, retains the foundation, because he does not doubt that he shall, at length, perceive that God is true. By which example we are taught, that there is no reason why we should despond, if, at any time, Satan should shake our faith; provided that the truth of God be not overthrown in our hearts. Meanwhile, when we see Abram, who, through so many years, had bravely contended like an invincible combatant, and had surmounted so many obstacles, now yielding, in a single moment, to temptation; who among us will not fear for himself in similar danger? Therefore, although we may have stood long and firmly in the faith, we must daily pray, that God would not lead us into temptation.