From the book:

Among the works of the reviver, Imām Muḥammad ibn ʿAbd al-Wahhāb (may Allāh have mercy on him), that have received notable acceptance from the scholars of the Ummah and seekers of knowledge, is this treatise before us, titled “The Treatise of the Three Fundamental Principles.”

The three fundamental principles that he (may Allāh have mercy on him) mentioned here are as follows:

The First Principle: Knowing the servant’s Lord.
The Second Principle: Knowing Islām with its proofs.
The Third Principle: Knowing our Prophet Muḥammad (May Allāh elevate his rank and grant him peace).
These three principles are the ones about which a person will be questioned when placed in their grave, and they are the ones mentioned by the Prophet (May Allāh elevate his rank and grant him peace) in his statement: “Indeed, the deceased hears the footsteps of his companions when they leave him, and two angels come to him and say: ‘Who is your Lord? What is your religion? Who is your Prophet?’ He says: ‘My Lord is Allāh, my religion is Islām, and my Prophet is Muḥammad (May Allāh elevate his rank and grant him peace).’ They will rebuke him and ask again: ‘Who is your Lord? What is your religion? Who is your Prophet?’ This is the final trial presented to the believer. As for the disbeliever and the hypocrite, it will be said to him: ‘What did you use to say about this man?’ He will say: ‘I do not know, I used to say what people used to say.’ Then it will be said to him: ‘You neither knew nor did you follow.’ Then he will be struck with a hammer of iron between his ears, and he will cry out in such a scream that everything near him will hear it except for humans and jinn.”