In the Name of Allah, most Compassionate, most Merciful

The issue of Ghairah is at the pinnacle of Islam.  Haya (Shame) and Ghairah is what our religion is all about.  Ghairah is the good jealousy, honor and possessiveness defined in Arabic which has no precise translation in English.  Allah has ghairah and far beyond anyone can imagine but Allah expects the believers to have Ghairah as well.  

Sahih Al-Bukhari Hadith 2.154 Narrated by Aisha
"O followers of Muhammad! By Allah! There is none who has more ghairah than Allah as He has forbidden that His slaves, male or female commits shameful sins. O followers of Muhammad! By Allah! If you knew that which I know you would laugh little and weep much."

As we know that looking at non-mahrem (non close relative members) is a shameful deed and thus to prevent this and other shameful sins, Allah has prescribed for the believing women to cover themselves and lower their gazes, as well for believing men to lower their gazes.  Because ghairah generally applies for women in Islam, Allah has told the believing women to cover themselves completely as not to be stared upon by non-mahrem or devils.

Sahih Al-Bukhari Hadith 8.829 Narrated by Al Mughira
Sa'd bin Ubada said, "If I found a man with my wife, I would kill him with the sharp side of my sword." When the Prophet heard that he said, "Do you wonder at Sa'd's sense of ghairah? Verily, I have more sense of ghairah than Sa'd, and Allah has more sense of ghairah than I."

So from all this, we see that how can a woman be allowed to uncover any part of the body, specially her face when outside of the house?  The Prophet's (S) wives covered their whole body and faces and his daughters.  Allah has more ghairah for the believing women then the Prophet (S) himself.  So how can any believing woman uncover herself outside the house or unveil her face among non-mahrem men?

Those scholars or Muslims who say that covering the face is not obligatory part of the hijab have challenged Allah's Ghairah.  On a record, one who challenges Allah is nothing but a disbeliever!  So all those Muslims and scholars from past and present who find conflicts and faults from the hadith that covering the face is not obligatory part of the hijab but a mere sunnah, are in fact hypocrites and liars.  Those scholars and people should be persecuted and purged from the ummah.  Those scholars do nothing but drop tons of clear evidences from Hadith but argue about Hadith that are not very clear.  This is one of the recognition and factor of Hypocrites. Prophet (S) has commanded the believers to drop anything that causes doubt and build on what is clear.