These are the three things, this real fear, this deep rooted fear in the heart of a person comes from three things:
That he knows about the crime he has committed,the sin, the violation of Allah’s order. He knows how disgusting it is, how filthy it is and how hated it is to Allah ta’alaa. Meaning he has knowledge of the texts from Allah and His Messenger (sallAllahu alaihi wasallam), describing the sin he did to know about its true status and how much Allah ta’alaa hates it. First step to have fear in your heart. To have knowledge of the sin that you are committing. Know how much it is a crime against Allah ta’alaa. How much He hates it.
To believe in the punishment, to believe sincerely and firmly with conviction that that punishment has been legislated for the people who do that thing. Allah ta’alaa has assigned it as something that is a just recompense for the crime committed. Then you know the reality. What is the key point in that second point? To have knowledge. Knowledge of the sin. Knowledge of the punishment that is waiting in line for the person who commits that sin.
He knows that he will never know whether he’ll be allowed to make tawbah for that sin or not. He is afraid that perhaps something will come between him and repentance to Allah ta’alaa. This fear, the reality, that I might not live to make repentance from this sin that I am about to do, or that I do, that I repeat often.
With these three things: Knowledge of the crime and how filthy it is, knowledge of the punishment for the people who commit this crime, reminding himself that he may not get a chance to repent from that sin.