Genesis 38:27-30 (ESV)
When the time of her labor came, there were twins in her womb. And when she was in labor, one put out a hand, and the midwife took and tied a scarlet thread on his hand, saying, “This one came out first.” But as he drew back his hand, behold, his brother came out. And she said, “What a breach you have made for yourself!” Therefore his name was called Perez. Afterward his brother came out with the scarlet thread on his hand, and his name was called Zerah.
Chapter 38 is a detour from the story of Joseph and his unlikely rise to a position from which he can save his whole family. This short, and seemingly unimportant, note about the birth of Perez and Zerah, is the end of that detour, and presumably the point. Perez is the great7-grandfather of David.
OK, but that genealogy could have been included without all the drama about Tamar. And what’s the point of the scarlet thread?
Actually, all this drama is needed to get the genealogy right. The midwife clearly sees this firstborn as the household heir (i.e., Er's firstborn). Her excessive attention to that feels like she was told to track it. If merely a child of Judah, it would be "behind" Shelah, and 1 Chronicles 4:21 notes many descendants of Shelah.
This means Judah confessed his sin fully to his household and also concluded that the firstborn counts as Er's as per Deuteronomy 25:5-6. This is essential to the genealogy of David and of Christ.
The fact that the thread is scarlet may signify something, but that’s not essential. Scarlet threads are mentioned in Exodus 26:1, 28:6 and Joshua 2:18 but the color isn’t specifically called special. Besides, it’s Zerah that ends up with the thread, not Perez.
What matters is why the midwife used the thread.
But Judah isn’t Israel’s firstborn, so how significant can this be anyway? Therein lies a deeper lesson.
While the culture emphasizes genealogy, and the Bible includes copious accounts of it, God makes a point of not following the “rules.” All this copious accounting serves to precisely record how God’s plans ignore (even mock) man’s rules.
This is yet another way that ancient scripture looks supernaturally wise in modern light. Judah’s the forth born of Jacob. Jacob’s the second born of Isaac. David will be the eighth son of Jesse. The LORD obviously doesn’t think much of man’s rules about firstborns.
Scripture will go on to record women as the first eyewitnesses of the resurrection, mocking the first century culture’s disrespect for the testimony of women.
People who are a generation ahead of their time are counted as super wise. The Bible’s a hundred generations ahead of its time.
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