There are two different answers to this question depending on what precisely is mean be the word evil.
First, if the word evil means moral evil, i.e. evil acts that human beings do that they should not do, then the answer is that there is evil in the world because men disobey God’s law. The question itself implies that there is some accepted code of moral absolutes. Such a code must come from an ultimate authority, a final court of appeal. This code cannot be a relative standard if the question is to have any real meaning because if there were no absolute standard that defined right and wrong evil would merely be what individual people chose to regard as evil, in other words what they personally do not like. And since men disagree all the time about what they consider is morally evil, this code cannot come from man, since it would in that case not be absolute, but relative. It must therefore come from somewhere else. If there is no such absolute standard of morality above man, then there is no evil at all, only different individual preferences, practices that some people like and regard as good and others don’t like and regard as bad. The question “why is there evil in the world?” presupposes that there must be some ultimate and absolute definition of good and evil, right and wrong, that is beyond man.
The source of this ultimate standard of right and wrong is always the source of a society’s law. Find the ultimate source of the law in any society and you have found that society’s God. This God may be a personal spiritual being or an ideology or institution, e.g. the State, but whoever or whatever is the final arbiter of right and wrong in a society is the God of that society. God in this sense is the person, ideology or institution that gives ultimate meaning to life. Evil is always defined by this ultimate authority, and is therefore always a religious question, a question of who God is.
Christianity teaches that the God of the Bible, as the Creator of heaven and earth, is the true God and that he is the one who defines good and evil. There is evil in the world, therefore, because people disobey God, who is the ultimate authority, the final arbiter of right and wrong, good and evil. If all men obeyed God there would be no evil in the world. Furthermore, men are without excuse for their evil deeds, i.e. their disobedience to God, because God has revealed his will, his definition of right and wrong, in his law. The answer to the question therefore is that there is evil in the world because men disobey God’s law.
If it is denied that anyone has the right to determine what right and wrong are, i.e. if we deny there is a God, then the logical conclusion is absolute anarchy. But of course even this presupposes an ultimate standard, namely, the individual himself, who becomes, as a result of his rejection of any authority above himself, his own ultimate authority, i.e. his own God. But anarchy of this sort is a minority sport, and it seldom exists in society (though doubtless individuals within society claim to believe in it). What actually happens in a society where the existence of a personal God is denied and it is believed that individuals make their own moral standards up as they go along is that this ultimate authority is turned into an abstract idea that is incarnated in the State. Since ultimate authority has to reside somewhere, and it cannot reside in each man, because men disagree about what is right or wrong, ultimate authority is attributed to the State. The State therefore becomes the ultimate authority in life, beyond which there is no appeal. In other words the State become God and sets about controlling the life of men and society, eventually in the most minute detail, according to its own definition of right and wrong, and in the process of course men loose their individual freedom.
Only Christianity offers a real alternative to these problems. Since God’s law is the ultimate standard, the ultimate definition of right and wrong, good and evil, there is a law above State law that guarantees man’s freedom under God’s law, and to which State law must conform. This was the ideal of the Rule of Law that governed the development of British justice and also the development of our understanding of the role of the State—until recent times of course, when this view has gone into decline as a result of the decline of the Christian faith in society, and along with this there has been a co-ordinate growth in the powers and influence of the State in our lives. This is because if God is denied as the source of ultimate authority someone or something else must take his place, and this is usually the State.
The point is that individual men and society at large have need of an ultimate arbiter of right and wrong, an ultimate standard. This arbiter will either be the God of the Bible or some other God, whether personal or abstract. In a society where the law of God is seen as being ultimate, all man-made law must conform to it, and this was traditionally the case in English common-law, a basic principle of which was that “Any law is or of right ought to be according to the law of God.” Accordingly, at her coronation in 1953, the Queen swore an oath to “maintain the Laws of God and the true profession of the Gospel.”
Second, if the word evil is being used in this question to mean natural catastrophes, for example earthquakes, typhoons and tsunamis that kill many people, the answer is that the Creation has been subjected by God to a curse because of man’s fall into sin (Genesis 3:17–19). As a consequence mankind has become subject to the hardships of life and death because of his sin. In other words mankind’s general subjection to natural disasters like earthquakes and typhoons is part of God’s judgement on mankind for his sin. This does not mean, however, that such disasters should necessarily be seen as a particular judgement of God for the individual sins of those people who experience such catastrophes. Rather, it is that mankind generally is subject to a world that labours under God’s curse for man’s sin.
However, God has also provided an atonement for man’s sin in the life, death and resurrection of the Lord Jesus Christ. Through faith in Christ man can escape the judgement of God on his sin and be reconciled to God. For those who put faith in the Lord Jesus Christ the resurrection will bring deliverance from the whole of this curse for man’s sin. The apostle Paul put it like this: “For the creature [i.e. Creation] was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope, because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God. For we know that the whole creation groaneth and travaileth in pain together until now” (Romans 8: 20–22). There is also this to consider: earthquakes happen, we know this, but as someone pointed out, men choose to live near the San Andreas Fault.
Ultimately the answer to both these versions of the question comes to the same thing, namely, that the cause of evil in the world is man’s sin and rebellion against God, i.e. his refusal to obey God’s law (the Bible defines sin as the transgression of God’s law—1 John 3:4). The important point to remember, however, is that the question itself presupposes a world of moral order, purpose and meaning, and the existence of moral absolutes, which would be impossible if there were no God. If there is no God and the world evolved by chance there is no purpose in life and therefore no meaning, and if there is no purpose and meaning in life there are no values that are absolute. In this case there is no such thing as good and evil, only chance happenings and personal preferences. The concept of evil presupposes that there is a God whose law defines good and evil and that mankind should obey that law.
Answered by Stephen Perks