Genesis 20:12—If incest is condemned, why did Abraham marry his sister?
Problem: Abraham admitted here that Sarah his wife was really his “sister” (cf. Gen. 17:15–16). Yet incest is denounced in no uncertain terms in many passages (cf. Lev. 18:6; 20:17). Indeed, the Lord declared, “Cursed is the one who lies with his sister, the daughter of his father or the daughter of his mother” (Deut. 27:22).
Solution: Abraham was not beyond sin, as his lie about Sarah to king Abimelech reveals (Gen. 20:4–5). And Abraham did admit that Sarah was “the daughter of my father, but not the daughter of my mother; and she became my wife” (Gen. 20:12). However, even granting this, there is no real proof Abraham violated any law for two reasons. First, the incest laws were not given by Moses until some 500 years after Abraham. So he surely could not be held responsible for laws that had not yet been promulgated. Second, the terms “sister” and “brother” are used with great latitude in the Bible, just as the terms “father” and “son.” Jesus, for example, was the “son” (i.e., descendant) of David (Matt. 21:15). “Sister” means a near relative, but it does not as such indicate the degree of nearness we understand by the word “sister.” Lot, Abraham’s nephew, is called a “brother” (Gen. 14:12, 16). Likewise, “daughter” can mean granddaughter or great granddaughter.
Considering the age to which Abraham lived (175, Gen. 25:7), it is possible that he married only a granddaughter on his father’s side, or even a niece or grand niece. In any event, there is no proof that Abraham’s marriage to Sarah violated any existing incest law. But if it did, the Bible simply gives us a true record of Abraham’s error. When God called Sarah Abraham’s “wife” (Gen. 17:15), He was not legitimizing any alleged incest, but merely stating a fact.
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This excerpt is from When Critics Ask: A Popular Handbook on Bible Difficulties (Wheaton, Ill.: Victor Books, 1992). © 2014 Norman Geisler and Thomas Howe. All rights reserved. Used by permission. Click here to purchase this book.