Levirate marriage is when the brother of a deceased man is obliged to marry his brother's widow. It was practiced in the Ancient Near East, Asia, India, etc.
Genesis 38:
That was Onan's levirate duty.
That wasn't masturbation but coitus interruptus.
The Lord put Onan to death because he refused to impregnate his dead brother's wife.
I agree. God promised Abraham that there would be a national Israel, his biological posterity to carry on the faith of God in the form of a national model for the world. God began this process of national development by providing 12 sons, as well as a daughter, to Jacob.
It is important to recognize that God was not just interested in Abraham's biological posterity, but also in the expression of faith being produced within the family. Abraham was chosen not just for his DNA, but more, for his responsibility in reproducing faith within his family. God was interested in family responsibility, as well as obedience within the family.
These children, including all 12 sons, were a vital part of God's development of faith in the nation. The daughter could conceivably carry Abraham's faith into other nations by marrying outside of the Hebrew culture, assuming it was not pagan. But the sons had to be committed to developing faith in their own nation to preserve faith there.
So their own children became vital in the production of the kind of national diversity God wanted for the nation as it developed faith. All 12 sons of Israel were viewed, by God, as a part of Abraham's national contribution to faith in God's word, producing children in the image of God.
Those who failed to model faith properly could be cut off, as Er was. But in this case it was nevertheless important to replace Judah's first-born son by providing a grandchild who would succeed where Er had failed. Why didn't God just rely on other sons of Judah to continue the tribal development of Judah?
It may have been because Er was a firstborn son of Judah, which became important under the Law as Israel's commitment to put God first in the matter of faith. The firstborn son represents the need of man to devote all that he has to the purposes of God.
His "firstborn" is his prioritized commitment to the things of God, just as Abraham was asked by God to commit his son of promise, Isaac, to God. To give your firstborn to faith is making faith the priority in your life. You don't give to yourself first, and then later give to God.
Onan's failure to cooperate with God's word and put God first before his own family was the sin. He didn't want to serve God's interests first, but his own.
God didn't just want Onan to give his own children to God. He wanted Onan to give to God first in his life by providing for Judah what Er failed to give.
It was *Judah's gift* that Onan was obligated to respect. It was his *father* that he was committed to honor and to serve on behalf of God's word. Onan's own family inheritance came second.
Nevertheless, God still wanted to use Onan, because he could still repent and cooperate with God in providing his father Judah with a reproduction of his firstborn son, an offering of faith to God. Onan was still capable of carrying on Judah's mission of biological expansion and putting God first above all other interests.
