Background and Context:
Onan, a figure in the Book of Genesis, is the second son of Judah and the brother of Er and Shelah. His account is primarily recounted in Genesis 38, which provides insight into the customs and familial obligations of the time, particularly the practice of levirate marriage. This practice, later codified in the Mosaic Law (Deuteronomy 25:5-10), required a man to marry his deceased brother's widow to produce offspring in the deceased brother's name, ensuring the continuation of the family line and the preservation of property within the family.

Biblical Account:
The narrative of Onan is found in Genesis 38:6-10. Judah, Onan's father, arranged a marriage between his firstborn son, Er, and a woman named Tamar. However, Er was "wicked in the sight of the LORD," and the LORD put him to death (Genesis 38:7). Following Er's death, Judah instructed Onan to fulfill his duty as a brother-in-law to Tamar by marrying her and raising offspring for his deceased brother.

Genesis 38:8-9 states: "Then Judah said to Onan, 'Sleep with your brother’s wife and fulfill your duty to her as a brother-in-law to raise up offspring for your brother.' But Onan knew that the offspring would not be his; so whenever he slept with his brother’s wife, he spilled his semen on the ground to avoid providing offspring for his brother."

Onan's Sin:
Onan's actions were a direct violation of the levirate marriage duty. By deliberately preventing conception, Onan not only failed to honor his brother's memory and legacy but also acted out of self-interest, seeking to preserve his own inheritance. His refusal to fulfill this familial obligation was seen as an act of disobedience and selfishness, which was "wicked in the sight of the LORD" (Genesis 38:10). Consequently, the LORD put him to death as well.

Theological Implications:
Onan's sin is often discussed in the context of disobedience to God's established order and the importance of family responsibilities. His actions are viewed as a breach of trust and duty, highlighting the seriousness with which God regards familial obligations and the continuation of the covenant line. The narrative underscores the principle that God's commands are to be followed with integrity and faithfulness, and that self-serving actions contrary to divine instructions can lead to severe consequences.

Cultural and Historical Considerations:
The practice of levirate marriage was integral to the social and familial structures of ancient Israel. It ensured the protection and provision for widows, who were vulnerable in a patriarchal society, and maintained the inheritance within the family. Onan's refusal to comply with this custom not only jeopardized Tamar's future but also threatened the stability and continuity of Judah's lineage.

Moral and Ethical Lessons:
Onan's account serves as a cautionary tale about the dangers of selfishness and the neglect of duty. It emphasizes the importance of honoring commitments and the potential repercussions of failing to do so. The account also reflects the broader biblical theme of God's justice and the expectation of righteousness in personal and communal relationships.