Why Did God Kill Onan? The Sin of Onan in Genesis 38

Onan appears in Genesis 38. His older brother Er died, and their father Judah told Onan to marry Er's widow, Tamar, and father a son who would carry on Er's name. Onan took the marriage but deliberately prevented Tamar from conceiving, because the child would not count as his. Genesis says "the thing which he did displeased the LORD," and God took his life. The text's main emphasis is on Onan's selfish refusal of a duty to his brother and to Tamar.

Here is what the passage says, and how it has been read.

What Happened

Judah's firstborn son, Er, died childless. Judah then turned to his second son:

And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother. And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother. And the thing which he did displeased the LORD: wherefore he slew him also.

Genesis 38:8–10 (KJV)

The key phrases explain his motive: "the seed should not be his" and "lest that he should give seed to his brother." Onan did not want to produce an heir for Er.

The Duty He Refused

This custom, called levirate marriage, was later written into the law of Moses:

If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of an husband's brother unto her. And it shall be, that the firstborn which she beareth shall succeed in the name of his brother which is dead, that his name be not put out of Israel.

Deuteronomy 25:5–6 (KJV)

The duty had two purposes. It kept the dead man's name and inheritance in his family line. And it protected the widow, who in that world had little security without a husband or son. Boaz later did exactly this for Ruth, "to raise up the name of the dead upon his inheritance" (Ruth 4:10).

Why Was His Sin So Serious?

Interpreters point to several layers:

• Greed. Er was the firstborn. If Er had no heir, more of the inheritance would pass to Onan. A son for Er meant less for Onan.
• Deceit. Onan did not refuse openly. He took Tamar as his wife and used the marriage for himself while secretly defeating its purpose. Under the later law, a man could refuse openly and face public shame instead (Deuteronomy 25:7–10).
• Wronging Tamar. He left her childless and trapped, unable to remarry outside the family.
• Despising his brother. He chose to let his brother's name disappear.

Genesis doesn't list these reasons, but its own wording, "lest that he should give seed to his brother," points to the first and the last. The rest of the chapter, where Tamar takes matters into her own hands and Judah finally admits "She hath been more righteous than I" (Genesis 38:26), shows how badly she had been wronged. See Tamar and Judah in Genesis 38.

How Christians Have Read the Passage

The passage has been read in different ways:

• On contraception. Roman Catholic teaching has historically cited Onan's act in opposing contraception, and many Protestant writers did the same until the 20th century.
• On masturbation. The term "onanism" became a name for masturbation, though Onan's act was withdrawal during intercourse, not masturbation.
• On the levirate duty. Many modern interpreters, Protestant and Jewish, read the sin primarily as Onan's refusal to fulfill his duty, since the text gives that as his motive.

Christians who disagree on the wider questions can agree on what the text clearly says: Onan put his own gain above his brother's name and a vulnerable woman's future, and he did it deceptively.

What It Means for Us

Onan's story is short and uncomfortable, but its lesson is plain. Selfishness that hides behind an outward show of duty is serious in God's eyes, especially when it harms the vulnerable. The Bible's wider teaching on sexuality is that our bodies are not just our own: "ye are bought with a price: therefore glorify God in your body" (1 Corinthians 6:20).

And Genesis 38 does not end in judgment. Through Tamar's son Pharez came the line of King David and, in time, Jesus himself. God's purposes were not defeated by Onan's refusal.