An Act of Obedience
The hijab is an act of obedience to Allah and to his Prophet sallallahu 'alayhi wa sallam, Allah says in the Qur'an:
"It is not for a believer, man or woman, when Allah and His Messenger have decreed a matter that they should have an option in their decision. And whoever disobeys Allah and His Messenger, has indeed strayed in a plain error."
Allah also said:
"And tell the believing women to lower their gaze (from looking at forbidden things) and protect their private parts (from illegal sexual acts, etc) and not to show off their adornment except what must (ordinarily) appear thereof, that they should draw their veils over their Juyubihinna."
Juyubihinna: The respected scholars from as-salaf as-salih (righteous predecessors) differed whether the veil cover of the body must include the hands and face or not. Today, respected scholars say that the hands and face must be covered. Other respected scholars say it is preferable for women to cover their whole bodies.
The Hijab is IFFAH (Modesty)
Allah (subhanahu wa ta'ala) made the adherence to the hijab a manifestation for chastity and modesty. Allah says:
"O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks (veils) over their bodies (when outdoors). That is most convenient that they should be known and not molested."
In the above Ayah there is an evidence that the recognition of the apparent beauty of the woman is harmful to her. When the cause of attraction ends, the restriction is removed. This is illustrated in the case of elderly women who may have lost every aspect of attraction. Allah (subhanahu wa ta'ala) made it permissible for them to lay aside their outer garments and expose their faces and hands reminding, however, that is still better for them to keep their modesty.
The Hijab is Tahara (Purity)
Allah (subhanahu wa ta'ala) had shown us the hikma(wisdom) behind the legislation of the hijab:
"And when you ask them (the Prophet's wives) for anything you want, ask them from behind a screen, that is purer for your hearts and their hearts."
The hijab makes for greater purity for the hearts of believing men and women because it screens against the desire of the heart. Without the hijab, the heart may or may not desire. That is why the heart is more pure when the sight is blocked (by hijab) and thus the prevention of fitna (evil actions is very much manifested. The hijab cuts off the ill thoughts and the greed of the sick hearts:
"Be not soft in speech, lest he in whose heart is a disease (of hypocrisy or evil desire for adultery, etc) should be moved with desire, but speak in an honourable manner."
The Hijab is a Shield
The Prophet sallallahu 'alayhi wa sallam said: "Allah, Most High, is Heaven, is Ha'yi (Bashful), Sit'tir (Shielder). He loves Haya' (Bashfulness) and Sitr (Shielding; Covering)." The Prophet sallallahu 'alayhi wa sallam also said: "Any woman who takes off her clothes in other than her husband's house (to show off for unlawful purposes), has broken Allah's shield upon her." The Hadith demonstrates that depending upon the kind of action committed there will be either reward (if good) or punishment (if bad).
The Hijab is Taqwa(Righteousness)
Allah (subhanahu wa ta'ala) says in the Qur'an:
"O Children of Adam! We have bestowed raiment upon you to cover yourselves (screen your private parts, etc) and as an adornment. But the raiment of righteousness, that is better."
The widespread forms of dresses in the world today are mostly for show off and hardly taken as a cover and shield of the woman's body. To the believing women, however the purpose is to safeguard their bodies and cover their private parts as a manifestation of the order of Allah. It is an act of Taqwa (righteousness).
The Hijab is Iman (Belief or Faith)
Allah (subhanahu wa ta'ala) did not address His words about the hijab except to the believing women, Al-Mu'minat. In many cases in the Qur'an Allah refers to the "the believing women". 'A'ishah, the wife of the Prophet sallallahu 'alayhi wa sallam, addressed some women from the tribe of Banu Tamim who came to visit her and had light clothes on them, they were improperly dressed: Hijab is Haya' (Bashfulness).
There are two authentic Hadith which state: "Each religion has a morality and the morality of Islam is haya'." AND "Bashfulness is from belief, and belief is in Al-Jannah (Paradise)." The hijab fits the natural bashfulness which is a part of the nature of women.
The Hijab is Ghirah
The hijab fits the natural feeling of Ghirah, which is intrinsic in the straight man who does not like people to look at his wife or daughters. Ghirah is a driving emotion that drives the straight man to safeguard women who are related to him from strangers. The straight MUSLIM man has Ghirah for ALL MUSLIM women. In response to lust and desire, men look (with desire) at other women while they do not mind that other men do the same to their wives or daughters. The mixing of sexes and absence of hijab destroys the Ghirah in men. Islam considers Ghirah an integral part of faith. The dignity of the wife or daughter or any other Muslim woman must be highly respected and defended.