Halakhah do Izajasza 6:10

The root of this commandment does not need elucidation. The matter is known and revealed to all that this belief is the foundation of the religion, and that one who does not believe in it denies a fundamental principle and has no share and merit with Israel. And the content of the belief is that he fix in his soul that such is the truth and that a change of (about a change of) this in any way be impossible. And if he be asked about it, he will respond to any enquirer that his heart believes this and he will not assent to a change in it, even if they say they will kill him [as a result]. As all of this - when he takes the thing from the potential to actuality, by which I mean to say when he puts into the words of his mouth what his heart decides - strengthens and fixes the belief of the heart. And if one merits to rise in the levels of wisdom, 'his heart will understand and his eyes will see' with a sealed proof that this belief that he believed is true and clear - it is impossible that there be something besides this - then he will fulfill this positive commandment [in the best way].