Genesis 38:8

One marked exception was needed to keep up tribal inheritance in Israel. It existed elsewhere, but it applied to no other people as it did to them, and still less to a Christian: a Levirate, or brother-in-law, marriage. It came into force when a man died childless. Then his brother or next of kin was called to raise up offspring for the dead man. The aim was to hold the family line and the inheritance together. If the nearer kinsman refused, the widow had the right to take off his shoe in public and spit in his face.

The law made a merciful provision (Deut. 25:5-10): no man's family should die out while a brother lived to carry on the line. In Israel, to be childless was a disgrace. If a man's name was blotted out — his family dying out — people saw it as a special mark of God's displeasure... In His provision there seems, too, to be a recognition of that hope in the heart of every Hebrew woman: that through her the promise of "the woman's seed" might somehow be fulfilled. This came true literally in the family line kept alive through Ruth.

Judah had no regard for the ancient landmark set up by Abraham, who said, "you shall not take a wife for my son from the daughters of the Canaanites" (Gen. 24:3)... The way he chose his wife is suggested by the phrase "And Judah saw," much the same as Samson in later years... There was no thought of making a choice according to the mind of God.

Onan married Tamar but then refused to grant her the privilege of being the mother of Judah's heir. He was prepared to marry her and enjoy conjugal relations, but he wanted to hold back his strength. The sin that is so displeasing to God in Onan's case was not simply spilling his seed on the ground, but selfishness. He wanted her to remain a childless and despised widow, and his whole attitude was governed by this. It was not a single sin but a repeated one, for the word "when" in verse 9 should read "whenever."

His seed still continues on the earth, even though God's wrath rests on the first two sons. Their names speak: Er of "enmity," and Onan of "iniquity." The third son, Shelah ("sprout"?), speaks of divine power in resurrection, bringing life out of death. So a remnant is preserved.

"It is evident that our Lord was descended from Judah" (Heb. 7:14). But how? "Judah was the father of Perez and Zerah by Tamar" (Matt. 1:3). This is especially striking. God, in His great grace, rises above the sin and folly of man to bring about His own purposes of love and mercy... How plain it is that when we reach the close of the first chapter of Matthew, it is "God manifest in the flesh" we find. Man could never have devised such a genealogy.