What is the meaning of "bEi'lokim Ahalel Devar"?
Radak, Malbim: I trust in Him, and praise the word that He told me about the kingship (refer to 56:5:1:2).
Why did he say "bEi'lokim Ahalel Devar" and also "ba'Shem Ahalel Devar"?
Rashi (from Yerushalayim Brachos 9:5): I will praise for Midas ha'Din, and also for Midas ha'Rachamim.
Radak: The matter is repeated [to strengthen it, like Radak says below (57:2) and in over 100 other places].
Radak citing his father: I will praise You whether You judge me with Midas ha'Din, or with Midas ha'Rachamim. Shocher Tov - we find that Elokim is Midas ha'Din - "Elokim Lo Sekalel" (Shemos 22:27), and Hashem is Midas ha'Rachamim - "Hashem Hashem Kel Rachum v'Chanun" (Shemos 34:6).
Malbim: Afterwards, Gad ha'Navi told him to go to Eretz Yehudah, and promised him a second time. Therefore, it says again Batachti ba'Shem, i.e. Midas ha'Rachamim, for He sent His words via a Navi 1 .
Verse 12 says "bEi'lokim Batachti"! And also the first time, Hashem told him via a Navi! This requires investigation. (PF)