Allaah سبحانه و تعالى says in the Qur'aan,
Allaah سبحانه و تعالى has made this religion to be established in the earth. But there are many humans and jinn in this world who do not wish to bow down to the Creator and Lord. Not only that, there are many jinn and humans in this world who do not wish to see others bow down to Allaah سبحانه و تعالى and live virtuous lives. Due to this, there will always be a struggle between truth and falsehood. Shaytaan and his followers will never give up their fight against Islaam and faith and the Muslims are ever obligated to struggle against them. Allaah سبحانه و تعالى says:
History demonstrates that as individuals embrace Islaam, they are often opposed by the people closest to them, such as family and close friends. This was true in the case of the Companions رضي الله عنهم of the Prophet صلى الله عليه وسلم in Makkah - long before the Islamic state was established - and it is also true for many people who embrace Islaam today in non-Muslim societies. Hence, as individuals seeking to follow the way of life they find to be true, Muslims will be opposed.
Islaam is not a religion simply for the individual. Indeed, when it spreads, it is a kind of ideological revolution that seeks to root out all aspects of jaahiliyyah in society. This kind of change will most likely be met with some resistance by those known as "the powers that be." Their entire values and way of life may be threatened or changed under the guidance of Islaam and those who were leisurely enjoying the old, false way of life may not be content with any change in their power or their ability to fulfill their desires. Indeed, they will not be satisfied. As Sayyid Qutb رحمه الله has noted, "History tells us, however, that the jaahili society always chooses to fight and not to make peace. It normally attacks the vanguard of Islaam at its very inception, whether it be a few persons, or larger groups, or it may wait until this vanguard has become a well-established community. From Noah to Muhammad, peace be upon them all, without exception, this has been the response of jaahili society to the call of Islaam." 
Allaah سبحانه و تعالى is well aware of what forces oppose Islaam and the Muslims. Allaah سبحانه و تعالى says in the Qur'aan,
In His mercy, Allaah سبحانه و تعالى did not leave Muslims without any guidance concerning this matter. Certainly one of the purposes of the Qur'aan is to guide the Muslims in this encounter with their enemies - be they enemies to the individual himself or opponents of the Muslim Ummah as a whole. It is for this reason that Allaah سبحانه و تعالى clearly describes these enemies and all those criminals who oppose the message of the Prophet صلى الله عليه وسلم. Allaah سبحانه و تعالى says,
Allaah سبحانه و تعالى describes who they are, what their goals are, how they behave and how the Muslim may defend himself against them. This is something that the reader must realise when he intends to read the Qur'aan: Allaah سبحانه و تعالى is showing him how to make himself stronger and protect himself against his enemies. Allaah سبحانه و تعالى is showing him the path that he should follow in the encounters he will have to face.
Perhaps the most obvious example of this category of teachings in the Qur'aan deals with Shaytaan. Allaah سبحانه و تعالى describes the source of the conflict between Shaytaan and humans. Allaah سبحانه و تعالى states,
Allaah سبحانه و تعالى also discloses to the believers what Shaytaan has in store for them:
The Qur'aan also depicts the ways and means of the Shaytaan; one such way is appearing as a sincere adviser and well-wisher, as was the way he approached Adam عليه السلام and Eve عليها السلام, although Allaah سبحانه و تعالى had warned them about Shaytaan (as He warns the Muslims about him in the Qur'aan):
Finally, Allaah سبحانه و تعالى guides the believer to what may protect him from Shaytaan:
(Soorah Fussilat, 41:36)
All of this is a mercy from Allaah سبحانه و تعالى and, based on Allaah's سبحانه و تعالى wisdom, it is part of the Qur'aan. The reader of the Qur'aan must realise that Allaah سبحانه و تعالى is warning him against this great enemy.
But this is not the only enemy that Allaah سبحانه و تعالى clearly describes or talks about in the Qur'aan. Allaah سبحانه و تعالى also discusses the disbelievers, be they polytheists, Jews or Christians, and how they plot against the Muslims. Allaah سبحانه و تعالى warns the Muslims about taking them as close friends and allies. The hypocrites are also minutely described in the Qur'aan. This is because they are a great danger to the social fabric and unity of the Muslim Ummah. They are an enemy from within that many Muslims may not recognise. Hence, Allaah سبحانه و تعالى has devoted long passages of the Qur'aan to their ways and, in fact, one Soorah even bears their name.
Allaah سبحانه و تعالى also cautions the believer concerning his own soul (an-nafs) - an enemy concerning which most humans would have been oblivious if it were not for the revelation of Allaah سبحانه و تعالى and His guidance. In two places in the Qur'aan, Allaah سبحانه و تعالى states,
(Sooratul-Hashr, 59:9 and Sooratut-Taghaabun, 64:16)
Allaah سبحانه و تعالى is telling the believer that if he is not cautious concerning what his own soul sometimes desires or leads himself to, he can easily take himself to his own destruction.
When the reader opens the Qur'aan, he should be aware that Allaah سبحانه و تعالى is telling him and warning him about his true enemies. When reading about Shaytaan, the disbelievers, the hypocrites and others, an alarm should sound in his mind telling him that Allaah سبحانه و تعالى is now describing those beings that are going to try to deviate him from the straight path. He should take not of how they behave and what he can do to defend himself and the Muslim society from them.