Why does it say "Bitchu va'Shem
Menachos 29b: Hashem is a fortress in this world and the world to come for all who trust in Him.
Radak #1: It is proper to trust in Him, for He is the strongest of all times. Tzur is an adjective. The same applies to "ha'Tzur Tamim Pa'alo" (Devarim 32:4).
Radak #2: Tzur is a noun. He is the strength of the worlds. This is like "b'Chei ha'Olam" (Daniel 12:7).
Mahari: Trust in Hashem, and not in the walls of Yerushalayim or the nations' love [for you], like your fathers did. This caused Galus!
Malbim: The verse says, do not trust in Shalom (refer to 26:3:2:1*), rather, in Hashem, and then it will be forever.
What is "Adei Ad"?
Radak #1: It is forever.
Radak #2, Targum Yonasan: Also "Adei" means forever. The matter is repeated, i.e. in all worlds.
Why does it say "b'Kah [Hashem] Tzur Olamim"?
Menachos 29b: Hashem created two worlds with the letters Yud-Kei - this world with Hei, and the world to come with Yud.
Targum Yonasan 1 : The strength of Hashem overpowers forever.
Radak: Kah is an adjective that indicates that the world is from Him. Hashem (Shem Havayah) is His name itself.
Malbim: There are two conditions to trust in someone. (a) He has the ability to fulfill the request of the one who trusts in him. (b) Constancy - his ability or desire to fulfill the request will never cease or change. Both of these are found only with Hashem. B'Kah Hashem - His existence is eternal. Tzur Olamim - He is the strength of all the worlds; they all are supported on Him, and via Him, they have endurance and strength. Automatically, His ability is over everything.
It seems that Rashi explains like this, just the Targum says to trust in the word of Hashem. (PF)
