Fearing Allah is a feeling carried within the hearts of all believers, and a feeling that should be held by all humanity. Allah's command in the Qur'an is clear:
You who believe! Fear Allah and let each self look to what it has sent forward for tomorrow. Fear Allah. Allah is aware of what you do. (Surat al-Hashr: 18)
Prophet Mohammed, also called attention to this issue in many of his sayings, reminding believers to always fear Allah, as Allah knows well what you do.
Allah knows what everybody keeps secret even within the most secret of secret recesses of their beings. He is very well aware of the types of misgivings the soul experiences and types of games that will be played out by the self because He is the creator of every soul and has given it boundless ability to do evil, and inspiration to avoid such evil so that it may be justly tried. Allah has also created Satan as part of this test and has granted him a set of specific qualities to be used in the pursuit of his objective.
Fearing Allah will enable person to behave as Allah requires, to work towards pleasing Allah, to protect oneself from temptations of the Satan, and the lower self with awareness of and resistance to their games.
Some of the Quran's verses, about fearing Allah:
So fear Allah, as much as you are able to, and listen and obey… (Surat at-Taghabun: 16)
You who believe! Fear Allah the way He should be feared and do not die except as Muslims. (Surah Al 'Imran: 102)
Those who fear their Lord in the Unseen will have forgiveness and an immense reward. (Surat al-Mulk: 12)
[People of intelligence are] …afraid of their Lord and fear an evil Reckoning. (Surat ar-Ra'd: 21)
[The Garden is for] those who fear the All-Merciful in the Unseen and come with a contrite heart. (Surah Qaf: 33)
[Prophets are] those who convey Allah's Message and fear Him and do not anyone but Allah. Allah suffices as a Reckoner. (Surat al-Ahzab: 39)
Although a believer fears Allah, he should never forget Allah's compassion, mercy and forgiveness, or that He is the One Who due to His generosity accepts repentance. This will in turn be the reason for possessing a sincere sense of hope, whilst experiencing genuine fear.
Therefore, a true believer will turn and supplicate to Allah in a spiritual state full of fear and hope:
Their sides eschew their beds as they call on their Lord in fear and ardent hope. And they give of what We have provided for them. (Surat as-Sajda: 16)