רבינו בחיי — שמע ישראל ה' אלהינו ה' אחד

Q — Why does the Shema need three names of Hashem? Wouldn't 'Hashem echad' suffice?

A — Rabbeinu Bachya runs through the alternative formulations and shows each fails. 'Hashem echad' alone — a gentile could agree, applying the name Hashem to his own avoda zarah (yes, our Hashem too is one). 'Hashem Eloheinu echad' alone — a gentile could say: yes, the elokim of Israel is indeed one Hashem; my elokim is also one Hashem. Only the three-part structure — Hashem (the universal name) // Eloheinu (specifically the God of Israel) // Hashem echad (and He is the only One bearing that name) — locks the meaning shut: the Hashem who is the God of Israel is the only Hashem there is, no other claim to the name is admissible. RBB then layers R' Elazar of Worms's reading on top: ה' = before the world existed, אלוקינו = within the world, ה' = after the world will be, אחד = in all worlds. The shema is therefore not a creed; it is a precision instrument whose three names close every loophole an opponent of monotheism might pry open.