Walā takūnū kalladhīna nasul-lāh fa-ansāhum anfusahum
And be not like those who forgot Allah so He made them forget themselves
(Sūratul Hashr, No.59, Āyat 19)
The consequence for those who forget Allah according to this verse is that they will forget themselves. Two questions that arise are:
1) What does it mean to forget Allah?
2) How can human beings forget themselves?
The answers are quite simple. To forget Allah is to be oblivious of the fact that Allah is the Creator and controls all things. That He is present everywhere and is watching all things. That everything is being recorded and there will be an eventual accounting. The awareness of this omnipresence of Allah creates a God consciousness or taqwā that does not allow the human being to forget the purpose of his creation. It helps him remain on track and live up to the potential he has a vicegerent of Allah. To forget Allah is to forget his primordial identity
Tafsīr-e Namūne explains that forgetting Allah results in a person being immersed in material pleasures only. This diminishes his humanity and prevents him from attaining the spiritual heights that are possible for him. It also makes him oblivious of the fact that Allah is the Master planner and controller. He thinks he is independent and it breeds arrogance in him. One of the biggest losses for the human being is to forget his humanity and his identity. It robs of him of all the worthiness and potential that are part of his essence and through which God has made him superior to all other creation. Remembering Allah on the other hand enlivens and strengthens that identity.
Imam Ali (a) talks about the effects of remembering Allah:
There are some people devoted to the remembrance (of Allah) who have adopted it in place of worldly matters so that commerce or trade does not turn them away from it. They pass their life in it. They speak into the ears of neglectful persons warning against matters held unlawful by Allah, they order them to practice justice and themselves keep practicing it, and they refrain them from the unlawful and themselves refrain from it. It is as though they have finished the journey of this world towards the next world and have beheld what lies beyond it. Consequently, they have become acquainted with all that befell them in the period during their long stay therein and the Day of Judgement fulfils its promises for them. Therefore they removed the curtain from these things for the people of the world till it was as though they were seeing what people did not see and were hearing what people did not hear (Nahjul Balāgha, Sermon 220)
Human beings have a fitrah or natural inclination to remember Allah. Note that the verse says those who forget Allah, implying that they know Him but forgot Him. You can only forget something you knew before. Sometimes the forgetting is short lived and the person returns back to the remembrance of Allah. But sometimes it is for long periods of time and can even become permanent.
Factors that take them away from this natural inclination to remember God include:
a) The whisperings of Shaytān
b) Attachment to the world
c) Committing sins
If the human being is vigilant and catches himself whenever he tends to forget Allah, he is quick to come back. The Quran says about such people: And those who when they commit an indecency or do injustice to their souls remember Allah and ask forgiveness for their faults (Q 3:135). And in another place: When those who are God conscious are touched by a visitation from Shaytān they remember [Allah] and behold, they perceive. (Q 7:201)
Be constantly alert about the remembrance of Allah. When you find you have forgotten Him for a while recite this verse and bring yourself back to Him. You do not want to be from those who lose their humanity and identity.
Sources:
Imam Ali bin Abu Talib, Nahjul Balāgha;
Āyatullāh Nāsir Makārim Shirāzī (ed.), Tafsīr-e Namūneh;
http://www.islamquest.net/fa/