Sefer Ha’mitzvos Ha’Katzar (the concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
2. It is a positive commandment to connect with the blessed G-d and to believe with complete faith that He is one without any partner.
As it states in Devarim (Deuternomy) 6:4: “Listen Israel, Hashem is our G-d, Hashem is One” – this is the main principle of faith. After the first knowledge that Hashem exists, it is necessary to believe with complete faith that He is one with the utmost degree of unity, without a body, no concepts of a physical being can apply to Him, nothing that affects physical beings can affect Him, there is no second being like Him, and without Him there is no G-d. Men and women are obligated in having this faith at all times and at every moment.
By Rabbi Yaacov Seltzer