Where did Hashem swear so to David?
Radak #1: Hashem's word is [like] His Shevu'ah. 1
Radak #2: Hashem said so twice - in Gilgal (after Sha'ul failed to wait for Shmuel), and after the episode with Amalek "Bikesh Lo Hashem Ish ki'Lvavo", "v'Nasnah l'Re'echa" 2 (1:13:14, 1:15:28). Saying "yes" twice, or "no" twice, is like a Shevu'ah (Shevu'os 36a).
Radak (Tehilim 89:36): It says "Im l'David Achazev" (89:36) - "Im" is an expression of a Shevu'ah.
Perhaps this is because a Shevu'ah equates fulfillment of the matter to the Emes of Hashem (R. Bechayei Shemos 20:7). However, it is not totally like a Shevu'ah. A decree for bad can be nullified, but not if it was sealed with a Shevu'ah (Malbim Shmuel 1 3:8, citing Mahari)! (PF)
I do not understand. These verses do not mention David. Hashem could fulfill them via someone else! (PF)
Why did he repeat 'Ki' - "Ki Ka'asher Nishba
Malbim: He saw (a) that this (insult from Ish Boshes) is from Hashem, so that David will reign, and (b) it is as if Hashem swore that I will do so (make David reign).
