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Both Jacob and Onan were directed by Jewish law to raise up children when each of their own childless brothers had died. Jacob obediently became the father of Joseph, but Onan disobediently became the father of the sin called "onanism."
On today's "Download," the two genealogies of Jesus were discussed. Chapter one of Matthew's Gospel starts with Abraham and counts down the father's lineage to Jesus. This genealogy lists Jacob as the father of St. Joseph. Verse 16 reads: "Jacob begot Joseph." This means St. Joseph's biological father is Jacob.
But turning to chapter three of Luke's Gospel, we see the genealogy starting with Jesus and counting up to Adam, listing Heli as St. Joseph's father. Verse 23 reads: "Joseph, who was of Heli."
The Challoner commentary given in the Douay Rheims Bible explains: "Heli and Jacob were brothers. Heli who was the elder, dying without issue, Jacob, as the law directed, married his widow: In consequence of such marriage, his son St. Joseph was reputed in the law the son of Heli."
In other words, Matthew lists Jacob as the biological father of St. Joseph, while Luke lists Heli as the legal father of St. Joseph.
This all is rooted in God's directive to Moses given in Deuteronomy 25:5–6:
When brethren dwell together, and one of them dies without children, the wife of the deceased shall not marry to another: But his brother shall take her, and raise up seed for his brother: And the first son he shall have of her he shall call by his name, that his name be not abolished out of Israel.
A man who disobeyed this directive was punished with public humiliation, as Deuteronomy 25:7–10 recounts:
But if he will not take his brother's wife, the rejected woman will say, "My husband's brother refuses to raise up his brother's name in Israel: and will not take me to wife." Next, before the elders, the woman shall take off his shoe from his foot, and spit in his face, and say, "So shall it be done to the man that will not build up his brother's house: And his name shall be called in Israel, the house of the unshod."
This all seems a bit outlandish to us, but the punishment for this offense stopped at public humiliation and wasn't punishable by death.
So Jacob obeyed the law in marrying his deceased brother's widow in order to father children on behalf of his deceased brother Heli.
This Mosaic law he obeyed actually began as a tribal law centuries earlier in the time of the patriarchs. Chapter 38 of Genesis recounts how Onan, in a similar situation, had an older brother named "Her" who had married Thamar but then died childless. At the death of his brother, Onan was directed by the patriarch Juda, "Go into thy brother's wife and marry her, that you may raise seed to your brother."
Unlike Jacob in the New Testament, Onan refused, as verse 9–10 recount:
He, knowing that the children should not be his, when he went into his brother's wife, spilled his seed upon the ground, lest children should be born in his brother's name. And therefore the Lord slew him, because he did a detestable thing.
In moral theology, "onanism" means contraception. In his 1930 encyclical "Casti Connubii," Pope Pius XI's exegesis of this incident teaches: "As St. Augustine notes, intercourse even with one's legitimate wife is unlawful and wicked where the conception of the offspring is prevented. Onan, the son of Juda, did this and the Lord killed him for it."