Shmot [Exodus] 20:2.
Know that God exists. That is the principal mitzva of the affirmative mitzvot — the basic imperative or commandment we use to build a relationship with G!d.
Mitzvot are transpersonal morals binding me to G!d. These morals inform interpersonal ethics that bind me socially to other people.
Transpersonal morality is designated by the Hebrew…
בּן אדם למקוֹם
Between G!d and Humanity
Interpersonal ethics is designated by the Hebrew…
בּן אדם לחברוֹ
Rambam does not preface the other taryag mitzvot this way. The very notion of a “principal” mitzva is not well-supported in Jewish tradition, for no single mitzva is more or less important than any other.
Here is an exception, for the very notion of mitzvot cannot be separated from the very notion of G!d.
Rambam’s mitzva is to know that G!d Exists, not to believe in G!d, and neither does he state that worship is mandatory, though see other imperatives in this collection for greater clarity.