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
1. It is a positive commandment to believe that G-d exists.
As it states in Shemos (Exodus) 20:2: “I am Hashem, your G-d, etc.” – and He, who is blessed and exalted, made all that is found and all of the worlds, from His strength and blessed desire. He is the one who watches over all things. And this is the pillar of our faith. And whoever does not believe this is a denier of the main principles and he does not have a portion or a merit within Israel. And we are required to discipline our souls and our might for this belief. The main thing is that one fixes his souls and this is the truth and that it is impossible for anything else.
This mitzvah applies at all times for men and women.