In The Name of Allaah, The Most Merciful, The Bestower of Mercy

Allaah [The Most High] said:

وَإِذَا سَمِعُواْ مَآ أُنزِلَ إِلَى ٱلرَّسُولِ تَرَىٰٓ أَعۡيُنَهُمۡ تَفِيضُ مِنَ ٱلدَّمۡعِ مِمَّا عَرَفُواْ مِنَ ٱلۡحَقِّ‌ۖ يَقُولُونَ رَبَّنَآ ءَامَنَّا فَٱكۡتُبۡنَا مَعَ ٱلشَّـٰهِدِينَ

وَمَا لَنَا لَا نُؤۡمِنُ بِٱللَّهِ وَمَا جَآءَنَا مِنَ ٱلۡحَقِّ وَنَطۡمَعُ أَن يُدۡخِلَنَا رَبُّنَا مَعَ ٱلۡقَوۡمِ ٱلصَّـٰلِحِينَ

فَأَثَـٰبَهُمُ ٱللَّهُ بِمَا قَالُواْ جَنَّـٰتٍ۬ تَجۡرِى مِن تَحۡتِهَا ٱلۡأَنۡهَـٰرُ خَـٰلِدِينَ فِيہَا‌ۚ وَذَٲلِكَ جَزَآءُ ٱلۡمُحۡسِنِينَ

And when they (who call themselves Christians) listen to what has been sent down to the Messenger (Muhammad), you see their eyes overflowing with tears because of the truth they have recognised.  They say: ”Our Lord! We believe; so write us down among the witnesses. And why should we not believe in Allaah and in that which has come to us of the truth (Islaamic Monotheism)? And we wish that our Lord will admit us (in Paradise on the Day of Resurrection) along with the righteous people. So because of what they said, Allaah rewarded them Gardens under which rivers flow (in Paradise), they will abide therein forever. Such is the reward of good-doers.”5:83-85

Shaikhul Islaam Ibn Taymiyyah [rahimahullaah] said: Therefore, Allaah [Glorified be He] did not give a promise for reward in the Hereafter, except to those who believe in Muhammad [sallal-laahu-alayhi-wasallam].  They are the ones about whom it is stated:

[وَإِذَا سَمِعُواْ مَآ أُنزِلَ إِلَى ٱلرَّسُولِ تَرَىٰٓ أَعۡيُنَهُمۡ تَفِيضُ مِنَ ٱلدَّمۡعِ مِمَّا عَرَفُواْ مِنَ ٱلۡحَقِّ‌ۖ يَقُولُونَ رَبَّنَآ ءَامَنَّا فَٱكۡتُبۡنَا مَعَ ٱلشَّـٰهِدِينَ   – And when they (who call themselves Christians) listen to what has been sent down to the Messenger, you see their eyes overflowing with tears because of the truth they have recognised.  They say: ”Our Lord! We believe; so write us down among the witnesses]. [5:83]

The witnesses are those who testify to the Messengership of Muhammad [sallal-laahu-alayhi-wasallam]. They bear witness that none has the right to be worshipped except Allaah and that Muhammad [sallal-laahu-alayhi-wasallam] is the Messenger of Allaah.  So, they are the witnesses being spoken about.  Allaah said:

وَكَذَٲلِكَ جَعَلۡنَـٰكُمۡ أُمَّةً۬ وَسَطً۬ا لِّتَڪُونُواْ شُہَدَآءَ عَلَى ٱلنَّاسِ وَيَكُونَ ٱلرَّسُولُ عَلَيۡكُمۡ شَهِيدً۬ا‌ۗ

”Thus We have made you [true Muslims – real believers of Islaamic Monotheism, true followers of Prophet Muhammad and his Sunnah (legal ways)], a Wasat (just) (and the best) nation, that you be witnesses over mankind and the Messenger (Muhammad) be a witness over you”. 2:143

And for that reason, Ibn Abbaas [radiyallaahu-anhu] and other than him stated [about the statement in the ayah: [فَٱكۡتُبۡنَا مَعَ ٱلشَّـٰهِدِينَ – so write us down among the witnesses]-Meaning:, ”Write us down with Muhammad [sallal-laahu-alayhi-wasallam] and his Ummah”.

[Source: Al-Jawabus Saheeh Liman Baddala Deenal Maseeh 2/46. Section: Faslun Fee But’laan Al-Istidlaal Bil-Mutashaabih]