The basis of all faith and the taproot of all beginnings, that paves the way towards understanding the truth of the elements of the divine Torah, is the believe in the existence of God, the Blessed. Therefore the purpose of this section [of Ohr Adonai] is to substantiate aspects and knowledge of the Torah of God, the Blessed, and it is worthy of examination, this root, and how it can be stood on firm ground.
Yet, if the first command of the Torah were to be belief in the existence of God, the Blessed, the commander would be commanded. And it is the nature of God to only be the one commanding, not a thing that is commanded! Yet this is the famous mistake of those who counted as a positive commandment belief in the existence of God, the Blessed, and this mistake is that the commands do not stand alone, there is no system of commands without a known and recognized commander.

And for this, when you count believing in God as a commandment, you already assume the existence of a God to command you, which you already knew that you should believe in. And if you were to believe that a God commanded you to believe in a God, you’d have to know that a God commanded you to believe in a God! And it would go on forever, this infinite chain of things you’d need to believe in, and so the whole point would be destroyed. And that is why you should never count believing in the existence of God, the Commander, as a commandment.

Excerpted from Sefaria.