Mufti Muhammad Shafi' رحمه الله wrote, 

"The belief in the Hereafter, among Islamic doctrines, is the one whose role in history has been what is nowadays described as revolutionary, for it began with transmuting the morals and manners of the followers of the Holy Qur'aan, and gradually gave them a place of distinction and eminence even in the political history of mankind. The reason is obvious. 

Consider the case of those who believe that life in the physical world is the only life, its joys the only joys and its pains the only pains, whose only goal is to seek the pleasures of the senses and the fulfilment of physical or emotional needs, and who stubbornly refuse to believe in the life of the Hereafter, in the Day of Judgment and the assessment of everyone's deeds, and in the requital of the deeds in the other world. When such people find the distinction between truth and falsehood, between the permissible and the forbidden, interfering with the hunt for the gratification of their desires, such differentiations naturally become intolerable to them. 

Now, who or what can effectively prevent them from committing crimes? The penal laws made by the state or by any other human authority can never serve either as real deterrents to crime or as agents of moral reform. 

Habitual criminals soon grow used to the penalties. A man, milder or gentler of temperament or just timid, may agree to forego the satisfaction of his desires for fear of punishment, but he would do so only to the extent that he is in danger of being caught. But in his privacy where the laws of the state cannot encroach upon his freedom of action, who can force him to renounce his pleasures and accept the yoke of restraints? 

It is the belief in the Hereafter and the fear of Allaah سبحانه و تعالى, and that alone, which can bring man's private behaviour in line with his public behaviour, and establish a harmony between the inner state and the outer. For the God-fearing man knows for certain that even in the secrecy of a well-guarded and sealed room and in the darkness of night somebody is watching him, and somebody is writing down the smallest thing he does. 

Herein lies the secret of the clean and pure society which arose in the early days of Islam when the mere sight of a Muslim, of his manners and morals, was enough to make non-believers literally fall in love with Islaam. For true Faith in the Hereafter, certitude must follow oral affirmation." 

Ma'aariful Qur'aan, Vol. 1, pp. 111-113, commentary on Sooratul Baqarah, 2:4

See also: Fear Allaah Wherever You Are (Taqwa)