Good and evil are probably concepts that people have talked and thought about the most. Most individuals try to be "good", according to their own standards, and to protect themselves and their loved ones from the evil actions of others. It's indeed most important to behave in such a way as to avoid evil. But many who expend great efforts to do so may have never thought about how close evil lies to them. Some have never considered the facts presented to them in the Qur'an, and think that evil is evil only when it is openly seen or and clearly experienced. For example, everyone knows that revenge, anger, jealousy, selfishness and lying are evil. They recognize that a person is vindictive when they see his anger, and a liar when they hear him telling lies.
Besides this, when the word evil is mentioned, actions generally accepted as wrong come to mind: murder, theft, treating others unfairly and other breaches of basic behavioral norms. Because they avoid such behavior and criticize it in others, they may therefore regard themselves as far removed from evil in general. However, evil has insidiously hidden aspects that cannot be clearly seen—and which many may not want to accept as elements of evil.
The reason is that those who don't live according to the Qur'an have their own measures of truth and falsehood. They determine their concepts of good and evil on the basis of their own beliefs, thoughts, attitudes and social education. However, the source where we can learn the real meaning of good and evil is evident—the Holy Qur'an Allah has sent to teach people right and wrong and to bring them from darkness into Light, and our Prophet's (saas) commentary on the Qur'an… The Prophet Muhammad (saas) emphasized the importance of learning the Qur'an and teaching it to others, thus:
"The best among you [Muslims] are those who learn the Qur'an and teach it to others." (Sahih Bukhari)
In the Qur'an, Allah explains what good and evil are and shows what draws a person toward evil. The Qur'an tells us that true good resides in the behavior of those who fear Allah and observe the limits He has set: "... rather devoutness is possessed by those who guard against evil..." (Surat al-Baqara: 189)
Whatever happens to him, a person who fears Allah will behave in a way that will please Allah and in accordance with the Qur'an. At every moment of his life he will exhibit this commendable behavior. A person will be led to always think and act rightly if he knows that everything that happens around him occurs under Allah's control and that everything happens for a good reason—and if he considers that in the Afterlife, he will be confronted with everything he has done both openly and in secret. Therefore, to be able to live a good life in the true sense, a person must fear Allah, believe in the Afterlife and make it his purpose to win Allah's favor. According to the Qur'an, behavior that does not conform to these standards is not truly good.
The Qur'an defines good and evil. Acting without the fear of Allah, taking other deities beside Him, serving someone else besides Him and living to please others and to make gains from this earthly life—all this can lead a person into evil. People without faith are passionately attached to the values of this world, which distance them from a good moral life. They always think first about how they can get the most out of life and set this above everything else—which leads them into selfishness, meanness, ingratitude falsity, arrogance, lying, mercilessness and other such negative moral qualities.
Those who live according to this way of thought may still try to avoid some things they consider as evil. But their calculations are usually directed toward avoiding criticism from others, not falling afoul of society's judgments and not damaging their own chances for gain. While they may avoid questionable behavior in public, they do not restrain themselves in secret. If confident that they won't be seen or heard, they're quite content to engage in such behavior, since these people have no fear of Allah. Their measure in avoiding evil is not to be known by others, so that they won't lose respect and will be generally well thought of in society. If they're sure that these conditions are met, they don't restrain their tendency toward every kind of evil. Therefore, these people base their idea of right and wrong on not getting caught.
However, Allah tells us in the Qur'an to avoid evil both openly and in secret and explains what it is: "... others have acknowledged their wrong actions and mixed a right action with another which is wrong. It may well be that Allah will turn towards them. Allah is Ever-Forgiving, Most Merciful." (Surat at-Tawba: 102) To avoid such situations, people must first know where evil originates and do everything they can to uncover its source. This will root out deception, and those who now think it's acceptable to do evil in secret will be led to avoid such behavior. Otherwise, as Allah says in the Qur'an, the life of a person deceived to think that evil can remain secret will be quite different from that of a believer, both in life of this world and the Hereafter.
"Or do those who perpetrate evil deeds suppose that We will make them like those who believe and do right actions, so that their lives and deaths will be the same? How bad their judgment is!" (Surat al-Jathiyya: 21) It says in the Qur'an that those who think they are doing good yet actually whose efforts are wasted are the greatest losers in the Hereafter:
Say: "Shall I inform you of the greatest losers in their actions?' People whose efforts in the life of this world are misguided while they suppose that they are doing good. Those are the people who reject their Lord's signs and the meeting with Him. Their actions will come to nothing and, on the Day of Resurrection, We will not assign them any weight." (Surat al-Kahf: 103-105)