The foundation of foundations and firmest pillar of all wisdom is, To know that there is a First Being, that He caused all beings to be, and that all beings from heaven and earth, and from between them, could not be save for the truth of His Own Being.
Observe them faithfully, for that will be proof of your wisdom and discernment to other peoples, who on hearing of all these laws will say, “Surely, that great nation is a wise and discerning people.”
כאשר זכה שכל האדם לראות אמיתת מציאותו ית‘, מיד נכנס בו שמחת גיל אין קץ...
Jazon Ish, Emuná UBitajon (Fe y Confianza) 2:9
– La certeza sobre la existencia de D’os genera felicidad. Cuando el intelecto de la persona logra un claro reconocimiento de la verdad de la existencia de D’os, inmediatamente siente una desbordante e infinita felicidad…
Regarding whether or not it is our duty to rationally investigate on the unity of G-d, I will say as follows: For anyone who is capable of investigating on this and other similar matters through rational inquiry - it is his duty to do so according to his intelligence and perception.
The proof that "lay it to your heart" refers to intellectual investigation, is from what the following verse says: "And none lays it to his heart, neither is there knowledge nor understanding" (Isaiah 44:19). So too David urged his son: "And you, Solomon my son, know you the G-d of your father, and serve him with a perfect heart and with a willing soul; for the L-ord searches all hearts" (Chronicles 28:9).
...It is impossible to arrive at faith (emunah) other than by means of truth (emet) as in the passage from the Zohar: "Justice shall be the girdle of his loins, and faith (Isaiah 11:5) -- justice and faith are one and the same" (Zohar 3:198, Parashat Balak). And it is stated there, "It is called faith when truth is joined to it".
In the evening when we affirm God’s existence in the darkness of night, we give expression through emunah, faith, since we can not use the physical senses to affirm our belief in God
הלכות יסודי התורה. יש בכללן עשר מצות, שש מצות עשה, וארבע מצות לא תעשה. וזהו פרטן:
(א) לידע שיש שם אלוה.
Mishneh Torah Sefer Madda Yesodei haTorah chapter one
Note that the Rambam uses the word "to know", and not "to believe". The popular translation of Sefer HaMitzvot (the Rambam himself composed the text in Arabic, the Mishneh Torah being the only text he wrote in Hebrew) begins: “The first mitzvah is the commandment... to believe in God.”
Many other Sages have objected to the latter definition of the commandment. For example, in his text Rosh Amanah, Rav Yitzchak Abarbanel mentions two frequently asked questions:
a) How can the first commandment be to believe in God? He is the one who issued the commandments. Without belief in Him, there can be no concept of serving Him by carrying out His will.
b) How can one command belief? Belief is a state of mind and not an action that is dependent on a person's will.
By stating that the command is "to know" - i.e., to develop one's knowledge and awareness of God - both of these questions are answered: Though one believes in God, he must work to internalize that belief and make it part of his conscious processes. Furthermore, the intellectual activity necessary for this process of internalization is an act which can be required of a person. See Derech Mitzvosecha, mitzvas HaAmanat Elokut.
[Note also the Hasagot of the Ramban to Sefer HaMitzvot and the response of the Megillat Esther.Also, it is worthy to mention that Rav Kapach and other modern translators of Sefer HaMitzvot also translate the command there as "to know."'