The Marriage of Merab: King Saul, David, and the Divine Plan
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The source for this is found in the Tanach (Hebrew Bible), Samuel I, Chapter 18.
King Saul (Shaul HaMelech) offered his eldest daughter, Merab, to David as a wife, in exchange for him going out and fighting the wars of Hashem (God):
"Behold my elder daughter Merab, her I will give to you as a wife; only be a valiant son to me and fight the wars of Hashem" (Samuel I 18:17).
But in the end, the scripture tells us:
"And it was at the time when Merab, Saul's daughter, should have been given to David, that she was given to Adriel the Meholathite as a wife" (Samuel I 18:19).
In other words, Merab was given to another man—Adriel the Meholathite—even before David had the chance to marry her.
Why did this happen?
Chazal (our Sages of blessed memory) and the commentators discuss this:
Rashi (Samuel I 18:19): Saul delayed and dragged the matter out, and in the meantime gave her to Adriel, in order to cause David to stumble.
Radak: David did not rush to accept the offer because he saw himself as poor and of little importance in his own eyes ("Who am I... that I should be a son-in-law to the king?").
Malbim: It is possible that Saul never truly intended to give her to him, but used it as an excuse to send David into dangerous wars, hoping he would be killed.
Chazal (Sanhedrin 19b): David was indeed worthy of Merab, but Merab was given to Adriel so that she would bear sons from him who would eventually die, and afterwards Michal (her sister) would raise them. This connects to the famous legend regarding the seven sons of Saul who were hanged in the days of David (Samuel II 21).