There is no disagreement that it is lawful to curse the disbelievers. `Umar bin Al-Khattab and the Imams after him used to curse the disbelievers in their Qunut (a type of supplication) during the prayer and otherwise. As for cursing a specific disbeliever, some scholars stated that it is not allowed to curse him, because we do not know how Allah will make his end. Others said that it is allowed to curse individual disbelievers. For proof, they mention the story about the man who was brought to be punished repeatedly for drinking (alcohol), a man said, "May Allah curse him! He is being brought repeatedly (to be flogged for drinking).'' Allah's Messenger said:

«لَا تَلْعَنْهُ فَإِنَّه يُحِبُّ اللهَ وَرَسُولَه»

(Do not curse him, for he loves Allah and His Messenger).

This Hadith indicates that it is allowed to curse those who do not love Allah and His Messenger . Allah knows best.

﴿وَإِلَـهُكُمْ إِلَـهٌ وَحِدٌ لاَّ إِلَـهَ إِلاَّ هُوَ الرَّحْمَـنُ الرَّحِيمُ ﴾

(163. And your Ilah (God) is One Ilah (God ـ Allah), La ilaha illa Huwa (there is none who has the right to be worshipped but He), the Most Gracious, the Most Merciful.)

In this Ayah, Allah mentions that He is the only deity, and that He has no partners or equals. He is Allah, the One and Only, the Sustainer, and there is no deity worthy of worship except Him. He is the Most Gracious ـ Ar-Rahman, the Most Merciful ـ Ar-Rahim. We explained the meanings of these two Names in the beginning of Surat Al-Fatihah. Shahr bin Hawshab reported that Asma' bint Yazid bin As-Sakan narrated that Allah's Messenger said:

«اسْمُ اللهِ الْأَعْظَمُ فِي هَاتَيْنِ الآيَتَيْنِ

(Allah's Greatest Name is contained in these two Ayat):

﴿وَإِلَـهُكُمْ إِلَـهٌ وَحِدٌ لاَّ إِلَـهَ إِلاَّ هُوَ الرَّحْمَـنُ الرَّحِيمُ ﴾

(And your Ilah (God) is One Ilah (God ـ Allah), La ilaha illa Huwa (there is none who has the right to be worshipped but He), the Most Gracious, the Most Merciful.) and:

﴿الم - ذَلِكَ الْكِتَابُ لاَ رَيْبَ فِيهِ هُدًى لِّلْمُتَّقِينَ ﴾

(Alif-Lam-Mim. Allah! La ilaha illa Huwa (none has the right to be worshipped but He), Al-Haiyul-Qaiyum (the Ever Living, the One Who sustains and protects all that exists).) (3:1, 2)''

Then Allah mentions some of the proof that He is alone as the deity, that He is the One who created the heavens and the earth and all of the various creatures between them, all of which testify to His Oneness. Allah said:

﴿إِنَّ فِي خَلْقِ السَّمَـوَتِ وَالاٌّرْضِ وَاخْتِلَـفِ اللَّيْلِ وَالنَّهَارِ وَالْفُلْكِ الَّتِى تَجْرِى فِى الْبَحْرِ بِمَا يَنفَعُ النَّاسَ وَمَآ أَنزَلَ اللَّهُ مِنَ السَّمَآءِ مِن مَّآءٍ فَأَحْيَا بِهِ الاٌّرْضَ بَعْدَ مَوْتِهَا وَبَثَّ فِيهَا مِن كُلِّ دَآبَّةٍ وَتَصْرِيفِ الرِّيَـحِ وَالسَّحَابِ الْمُسَخَّرِ بَيْنَ السَّمَآءِ وَالأَرْضِ لآيَـتٍ لِّقَوْمٍ يَعْقِلُونَ ﴾

(164. Verily, in the creation of the heavens and the earth, and in the alternation of night and day, and the ships which sail through the sea with that which is of use to mankind, and the water (rain) which Allah sends down from the sky and makes the earth alive therewith after its death, and the moving (living) creatures of all kinds that He has scattered therein, and in the veering of winds and clouds which are held between the sky and the earth, are indeed Ayat (proofs, evidences, signs, etc.) for people of understanding.)