The first of the 613, and the ground everything else stands on. It is drawn from the opening line of the Ten Commandments — Shemot 20:2 — and it is the first mitzvah in the Rambam's Sefer HaMitzvot.

Notice what it asks. Not to feel, not to hope, but to know that there is a G-d. Simple to say, and the work of a lifetime to live.