In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Allaah [The Exalted] said: [سَنُلۡقِى فِى قُلُوبِ ٱلَّذِينَ كَفَرُواْ ٱلرُّعۡبَ بِمَآ أَشۡرَڪُواْ بِٱللَّهِ مَا لَمۡ يُنَزِّلۡ بِهِۦ سُلۡطَـٰنً۬اۖ – We shall cast terror into the hearts of those who disbelieve, because they ascribe partners to Allaah, for which He had sent no authority]. [Surah Aal Imraan. Aayah 151]
Imaam Ibnul Qayyim [rahimahullaah] said: The level of Shirk determines the level of fear, and the one who ascribes a partner to Allaah has the greatest fear and terror. Those who believe and do not mix their belief with Shirk have safety, guidance and success and the one who ascribes a partner to Allaah has fear, misguidance and wretchedness. [Ref 1]
Imaam As-Sadi [rahimahullaah] said that this fear is due to that which they took as partners to Allaah, such as those things they set up as equals to Allaah and the idols based on their vain desires and corrupt goals, for which they neither have proof nor evidence, and thus they cut themselves off the Protection of The One and Only True Deity, The Most Mercy. [Ref 2]
Read: Belief, Attitude And Behaviour Whether There’s a Lock down or Not? [Do Not Allow Shaytaan, Media Hype and Scare-mongering About Illness to Turn You Into a Paranoid Wreck]!
[Ref 1: Iddat As-Saabireen 166-167. Slightly paraphrased]
[Ref 2: Tafseer As-Sadi]. Slightly paraphrased]