Excerpts from entry for 19th Shevat in “HaYom Yom,” compiled by the Lubavitcher Rebbe:

In the words of Rambam [Maimonides],* “To know that there is a First Being, Who caused the existence of all beings… The knowledge of this principle is a positive command, as it is said, I am the Eternal your G-d.”

This is a mitzva [commandment] relating to mind and intellect… it is the duty of the mind and intellect to bring this faith to a level of knowledge and comprehension. This is the meaning of, “To know that there is a First Being;” the mitzva specifies comprehension and intellectual grasp, as written in Torah… “serve Him with a whole heart” [Chronicles I 28:9], and it is also written, “know this day [and consider it in your heart, that the L-rd, He is G-d in Heaven above, and upon the earth below; there is none else].” [Deut. 4:39]

*Mishneh Torah, Foundations of the Torah 1:1.